[stdlib]Properly promote stack buffer to heap buffer when necessary (5.0 cherry pick)

[lancep]

rdar://problem/45216800
https://bugs.swift.org/browse/SR-8939
Original PR: #20585

[lancep]

@swift-ci please test

[lancep]

@swift-ci please benchmark

[lancep]

@airspeedswift Don't let this interrupt your weekend/holiday plans too much. It would just be nice to get this in sooner rather than later.

[swift-ci]

Build comment file:

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
InsertCharacterEndIndex	145	160	+10.3%	0.91x
StringEqualPointerComparison	600	657	+9.5%	0.91x
Improvement
Hanoi	3779	3487	-7.7%	1.08x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
StringEqualPointerComparison	571	628	+10.0%	0.91x
InsertCharacterEndIndex	145	159	+9.7%	0.91x
Improvement
DictionaryKeysContainsNative	30	27	-10.0%	1.11x (?)

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Improvement
ArrayOfGenericPOD2	1179	1067	-9.5%	1.10x (?)
ArrayOfPOD	858	780	-9.1%	1.10x

How to read the data

The tables contain differences in performance which are larger than 8% and differences in code size which are larger than 1%.

If you see any unexpected regressions, you should consider fixing the
regressions before you merge the PR.

Noise: Sometimes the performance results (not code size!) contain false
alarms. Unexpected regressions which are marked with '(?)' are probably noise.
If you see regressions which you cannot explain you can try to run the
benchmarks again. If regressions still show up, please consult with the
performance team (@eeckstein).

Hardware Overview

  Model Name: Mac Pro
  Model Identifier: MacPro6,1
  Processor Name: 12-Core Intel Xeon E5
  Processor Speed: 2.7 GHz
  Number of Processors: 1
  Total Number of Cores: 12
  L2 Cache (per Core): 256 KB
  L3 Cache: 30 MB
  Memory: 64 GB

--------------