SIL: remove the private_external SILLinkage

[eeckstein]

The private_external linkage was a relict from the sil-serialize-all days. This linkage doesn't make any sense because a private function cannot be referenced from another module (or file, in case of non-wmo compilation).

This PR also contains another change: diagnose body-less private functions with an _silgen_name attribute

Functions with an @_silgen_name attribute can be declared without a body - for referencing externally defined C functions. But this does not make sense for private functions.
So far, this error was not diagnosed - and it didn't cause any problem, because such functions were generated with a private_external linkage.
With removing the linkage, it does not work anymore and would result in SILVerifier errors.

[eeckstein]

@swift-ci test

[eeckstein]

@swift-ci benchmark

[slavapestov]

I love these cleanups!

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - c6b608b2c6ca50e5d68bece9c5c37a10ff63ae79

[swift-ci]

Performance (x86_64): -O

Regression	OLD	NEW	DELTA	RATIO
NSStringConversion.Rebridge.Medium	151	168	+11.3%	0.90x (?)
NSStringConversion.Rebridge.Long	151	168	+11.3%	0.90x (?)
NSStringConversion.Rebridge.UTF8	376	418	+11.2%	0.90x (?)
NSStringConversion.Rebridge.LongUTF8	45	50	+11.1%	0.90x (?)
DataAppendArray	3300	3600	+9.1%	0.92x (?)
Improvement	OLD	NEW	DELTA	RATIO
NSError	145	134	-7.6%	1.08x (?)

Code size: -O

Performance (x86_64): -Osize

Code size: -Osize

Performance (x86_64): -Onone

Improvement	OLD	NEW	DELTA	RATIO
ArrayAppendGenericStructs	2070	1160	-44.0%	1.78x (?)

Code size: -swiftlibs

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: 8-Core Intel Xeon E5
  Processor Speed: 3 GHz
  Number of Processors: 1
  Total Number of Cores: 8
  L2 Cache (per Core): 256 KB
  L3 Cache: 25 MB
  Memory: 64 GB

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - c6b608b2c6ca50e5d68bece9c5c37a10ff63ae79

[eeckstein]

@swift-ci test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 8e0fad2629a8452a515edede2fd8a8fd77daa27e

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 8e0fad2629a8452a515edede2fd8a8fd77daa27e

[eeckstein]

@swift-ci test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - e526abc792714880c63a6ba35bc31bdc1df2cedc

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - e526abc792714880c63a6ba35bc31bdc1df2cedc

[eeckstein]

@swift-ci test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 70e96d48b9f4956e810b3f40c3d21f11266f9daf

[eeckstein]

@swift-ci test