The other part of rdar://problem/21444126. This is a little trickier since SIL doesn't track uses of witness tables in a principled way. Track uses in SILGen by putting a "SILGenBuilder" wrapper in front of SILBuilder, which marks conformances from apply, existential erasure, and metatype lookup instructions as used, so we can avoid emitting shared Clang importer witnesses when they aren't needed.
Swift SVN r29544
Since we weren't assigning region counters until we were emitting
function bodies, we would crash while walking the expressions in
default arguments. This assigns the counters earlier and fixes up the
ASTWalker to expect that.
Swift SVN r25445
This adds the -profile-coverage-mapping option to swift, and teaches
SILGenProfiling to generate mappings from source ranges to counters.
Swift SVN r25266
This adds the -profile-generate flag, which enables LLVM's
instrumentation based profiling. It implements the instrumentation
for basic control flow, such as if statements, loops, and closures.
Swift SVN r25155
