[Runtime] Eliminate stack frames in swift_retain and swift_bridgeObjectRetain on ARM64.

[mikeash]

Rearrange the slow paths a bit to make them tail calls, which allows the compiler to emit these functions without frames.

Clang is happy to emit frameless functions on ARM64 if no stack space is needed on all execution paths. However, when there's a fast path which doesn't need stack space, and a slow path which does, clang emits code that pushes a stack frame and then decides which path to take. This is fine, but it means we're paying more than we'd like to on the fast path.

We can work around that by manually outlining the slow path, and ensuring that it's invoked with a tail call. Then the original function doesn't need a stack frame on any path and clang omits the stack frame.

We tweak RefCounts::increment to return the object it's being called on, which allows swift_retain to tail-call it. We manually outline the objc_retain call in swift_bridgeObjectRetain, which allows the swift_retain path to be frameless.

rdar://101764509

[mikeash]

@swift-ci Please Apple Silicon benchmark

[mikeash]

------- Performance (arm64): -O -------

REGRESSION                                 OLD   NEW   DELTA    RATIO    
ObjectiveCBridgeFromNSString               630   685   +8.7%    **0.92x (?)**

IMPROVEMENT                                OLD   NEW   DELTA    RATIO    
StringAdder                                248   173   -30.2%   **1.43x (?)**
StringFromLongWholeSubstringGeneric        4     3     -25.0%   **1.33x (?)**
PrefixWhileAnySequenceLazy                 403   329   -18.4%   **1.22x (?)**
DictionaryBridgeToObjC_Bridge              6     5     -16.7%   **1.20x (?)**
ParseFloat.Float.Exp                       7     6     -14.3%   **1.17x (?)**
LessSubstringSubstring                     22    19    -13.6%   **1.16x (?)**
EqualSubstringSubstring                    22    19    -13.6%   **1.16x (?)**
EqualStringSubstring                       22    19    -13.6%   **1.16x (?)**
EqualSubstringSubstringGenericEquatable    22    19    -13.6%   **1.16x (?)**
EqualSubstringString                       22    19    -13.6%   **1.16x (?)**
LessSubstringSubstringGenericComparable    22    19    -13.6%   **1.16x (?)**
ConvertFloatingPoint.MockFloat64ToDouble   10    9     -10.0%   **1.11x (?)**
Hanoi                                      950   860   -9.5%    **1.10x (?)**
SequenceAlgosList                          330   300   -9.1%    **1.10x (?)**
Set.subtracting.Box.Empty                  13    12    -7.7%    **1.08x (?)**
DictionaryKeysContainsNative               14    13    -7.1%    **1.08x (?)**

------- Performance (arm64): -Osize -------

REGRESSION                                        OLD    NEW    DELTA    RATIO    
RawBufferCopyBytes                                11     13     +18.2%   **0.85x (?)**
ObjectiveCBridgeStubNSDataAppend                  1020   1110   +8.8%    **0.92x (?)**
ArrayAppendOptionals                              370    400    +8.1%    **0.93x (?)**

IMPROVEMENT                                       OLD    NEW    DELTA    RATIO    
StringAdder                                       247    191    -22.7%   **1.29x (?)**
EqualStringSubstring                              22     19     -13.6%   **1.16x (?)**
EqualSubstringString                              22     19     -13.6%   **1.16x (?)**
LessSubstringSubstring                            23     20     -13.0%   **1.15x (?)**
EqualSubstringSubstring                           23     20     -13.0%   **1.15x (?)**
EqualSubstringSubstringGenericEquatable           23     20     -13.0%   **1.15x (?)**
LessSubstringSubstringGenericComparable           23     20     -13.0%   **1.15x (?)**
StringToDataEmpty                                 400    350    -12.5%   **1.14x (?)**
StringToDataSmall                                 450    400    -11.1%   **1.12x (?)**
CharIteration_russian_unicodeScalars              3040   2720   -10.5%   **1.12x (?)**
SequenceAlgosList                                 330    300    -9.1%    **1.10x (?)**
CharIteration_ascii_unicodeScalars                2560   2360   -7.8%    **1.08x**
Set.subtracting.Empty.Box                         13     12     -7.7%    **1.08x (?)**
CharIteration_korean_unicodeScalars               3280   3040   -7.3%    **1.08x (?)**
CharIteration_tweet_unicodeScalars                4960   4600   -7.3%    **1.08x (?)**
String.replaceSubrange.ArrChar                    28     26     -7.1%    **1.08x (?)**
Set.subtracting.Box.Empty                         14     13     -7.1%    **1.08x (?)**
CharIteration_chinese_unicodeScalars              2280   2120   -7.0%    **1.08x (?)**
CharIteration_punctuatedJapanese_unicodeScalars   600    560    -6.7%    **1.07x (?)**
Set.isStrictSuperset.Seq.Box0                     15     14     -6.7%    **1.07x (?)**
CharIteration_japanese_unicodeScalars             3640   3400   -6.6%    **1.07x (?)**

------- Performance (arm64): -Onone -------

REGRESSION                                OLD     NEW     DELTA    RATIO    
StringToDataLargeUnicode                  1800    1950    +8.3%    **0.92x (?)**
NSArray.bridged.objectAtIndex             264     285     +8.0%    **0.93x (?)**

IMPROVEMENT                               OLD     NEW     DELTA    RATIO    
SortArrayInClass                          74803   62321   -16.7%   **1.20x (?)**
EqualSubstringSubstringGenericEquatable   25      21      -16.0%   **1.19x (?)**
EqualSubstringSubstring                   26      22      -15.4%   **1.18x (?)**
LessSubstringSubstring                    25      22      -12.0%   **1.14x (?)**
LessSubstringSubstringGenericComparable   25      22      -12.0%   **1.14x (?)**
EqualStringSubstring                      26      23      -11.5%   **1.13x (?)**
EqualSubstringString                      26      23      -11.5%   **1.13x (?)**
SequenceAlgosList                         3950    3540    -10.4%   **1.12x**
SortSortedStrings                         59      55      -6.8%    **1.07x**
SortStrings                               902     842     -6.7%    **1.07x (?)**

[mikeash]

@swift-ci please test

[mikeash]

@swift-ci please test

[mikeash]

@swift-ci please test