[TLS] Add a uText to our TLS

[milseman]

ICU's uText APIs offer a cross-encoding means of interacting with
grapheme breaking. Storing one on our TLS allows us to explore mised
encodings, and even for UTF-16 might be profitable if setText
mallocs/frees an internal uText (which seems to be the case). This
isn't hooked up to anything yet, but its highly likely we will be
using it soon and adding it now enables more rapid development of the
UTF-8 prototype, due to it's tie-in with the runtime.

[milseman]

@swift-ci please smoke benchmark

[milseman]

@swift-ci please test

[milseman]

@weissi, once this lands and gets incorporated into a toolchain, prototype development with non-fast-pathed-graphemes can continue.

[swift-ci]

Build comment file:

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
CStringLongAscii	3521	3273	-7.0%	1.08x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Improvement
ObjectiveCBridgeFromNSSetAnyObjectToString	83334	77161	-7.4%	1.08x
CStringLongAscii	3517	3285	-6.6%	1.07x

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Regression
ArrayOfGenericPOD2	1067	1179	+10.5%	0.91x (?)
ArrayOfPOD	783	858	+9.6%	0.91x

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

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