Add CalculateSymbolSizes in ObjectFilePECOFF::GetSymtab to ensure that...

Add CalculateSymbolSizes in ObjectFilePECOFF::GetSymtab to ensure that (nearly) all the symbols have sizes.

This fixes the `thread step-over` regression exposed by http://reviews.llvm.org/D16186 , which depends on the symbols having actual sizes.  Nine tests on Windows had started failing as a result.  They all work again with this fix.

Differential Revision: http://reviews.llvm.org/D16415

llvm-svn: 258429