swift234

Convert all external function definitions to declarations when compiling with -Onone.

This avoids that an unoptimized imported function is linked instead the optimized version from the stdlib.
rdar://problem/20485253

It gives considerable performance improvmenets for some benchmarks with -Onone. E.g.
PopFrontUnsafePointer: +281%
ArrayOfPOD: +92%
StrComplexWalk: +91%
ArrayOfGenericPOD: +61%
Several others are within the range of +10% to +30%.

For the implementation I added runSILPassesForOnone() in Passes.cpp.
Here we can add other optimizations for -Onone in the future.



Swift SVN r27206