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)'

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@220507 91177308-0d34-0410-b5e6-96231b3b80d8