swift234

Add @semantics("optimize.never") to disable optimizations of a specific function.

This commit adds a flag to disable optimizations on a specific functions. The
primary motivation of this patch is to allow the optimizer developers to reduce
testcasese by disabling optimizations of parts of the code without having to
recompile the compiler or inspect SIL. The annotations  "inline(never)"
and "optimize.none" can go a long way.

The second motivation for this patch is to allow our internal adopters to work
around compiler bugs.

rar://19745484

Usage:

@semantics("optimize.never")
public func miscompile() { ... }

Swift SVN r27475