[ABI] Eliminate witness table accessors.

Witness table accessors return a witness table for a given type's
conformance to a protocol. They are called directly from IRGen
(when we need the witness table instance) and from runtime conformance
checking (swift_conformsToProtocol digs the access function out of the
protocol conformance record). They have two interesting functions:

1) For witness tables requiring instantiation, they call
swift_instantiateWitnessTable directly.
2) For synthesized witness tables that might not be unique, they call
swift_getForeignWitnessTable.

Extend swift_instantiateWitnessTable() to handle both runtime
uniquing (for #2) as well as handling witness tables that don't have
a "generic table", i.e., don't need any actual instantiation. Use it
as the universal entry point for "get a witness table given a specific
conformance descriptor and type", eliminating witness table accessors
entirely.

Make a few related simplifications:

* Drop the "pattern" from the generic witness table. Instead, store
  the pattern in the main part of the conformance descriptor, always.
* Drop the "conformance kind" from the protocol conformance
  descriptor, since it was only there to distinguish between witness
  table (pattern) vs. witness table accessor.
* Internalize swift_getForeignWitnessTable(); IRGen no longer needs to
  call it.

Reduces the code size of the standard library (+assertions build) by
~149k.

Addresses rdar://problem/45489388.

Author: DougGregor

Darwin/Foundation-swiftoverlay/NSError.h

@@ -27,11 +27,9 @@ namespace swift {
 /// The items that ErrorObject.mm needs for bridging. The
 /// ERROR_BRIDGING_SYMBOL_NAME symbol will contain an instance of this struct.
 struct ErrorBridgingInfo {
-  const SWIFT_CC(swift) WitnessTable *(*GetCFErrorErrorConformance)();
-  
-  const SWIFT_CC(swift) hashable_support::HashableWitnessTable *
-    (*GetNSObjectHashableConformance)();
-  
+  const ProtocolConformanceDescriptor *CFErrorErrorConformance;
+  const ProtocolConformanceDescriptor *NSObjectHashableConformance;
+
   SWIFT_CC(swift) NSDictionary *(*GetErrorDefaultUserInfo)(const OpaqueValue *error,
                                                            const Metadata *T,
                                                            const WitnessTable *Error);

Darwin/Foundation-swiftoverlay/NSError.mm

@@ -17,11 +17,11 @@
 using namespace swift;
 
 // Declare the mangled Swift symbols that we'll be putting in the bridging info.
-extern "C" const SWIFT_CC(swift) WitnessTable *
-  MANGLE_SYM(So10CFErrorRefas5Error10FoundationWa)();
+extern "C" const ProtocolConformanceDescriptor
+  MANGLE_SYM(So10CFErrorRefas5Error10FoundationMc);
 
-extern "C" const SWIFT_CC(swift) hashable_support::HashableWitnessTable *
-  MANGLE_SYM(So8NSObjectCSH10ObjectiveCWa)();
+extern "C" const ProtocolConformanceDescriptor
+  MANGLE_SYM(So8NSObjectCSH10ObjectiveCMc);
 
 extern "C" SWIFT_CC(swift)
   NSDictionary *MANGLE_SYM(10Foundation24_getErrorDefaultUserInfoyyXlSgxs0C0RzlF)(
@@ -36,8 +36,8 @@
 
 // Define the bridging info struct.
 extern "C" ErrorBridgingInfo ERROR_BRIDGING_SYMBOL_NAME = {
-  MANGLE_SYM(So10CFErrorRefas5Error10FoundationWa),
-  MANGLE_SYM(So8NSObjectCSH10ObjectiveCWa),
+  &MANGLE_SYM(So10CFErrorRefas5Error10FoundationMc),
+  &MANGLE_SYM(So8NSObjectCSH10ObjectiveCMc),
   MANGLE_SYM(10Foundation24_getErrorDefaultUserInfoyyXlSgxs0C0RzlF),
   MANGLE_SYM(10Foundation21_bridgeNSErrorToError_3outSbSo0C0C_SpyxGtAA021_ObjectiveCBridgeableE0RzlF),
   &MANGLE_SYM(10Foundation26_ObjectiveCBridgeableErrorMp)