Disabling some MSVC warnings that are of questionable value. Note, these are...

Disabling some MSVC warnings that are of questionable value. Note, these are disabled by default in LLVM as well, so there is precedence.

C4146: 'unary minus operator applied to unsigned type, result still unsigned'
C4291: ''declaration' : no matching operator delete found; memory will not be freed if initialization throws an exception'
C4800: ''type' : forcing value to bool 'true' or 'false' (performance warning)'
llvm-svn: 220507