Explicitly request unsigned enum types when desired

The underlying type of all plain enums in MSVC is 'int', even if the
enumerator contains large 32-bit unsigned values or values greater than
UINT_MAX.  The only way to get a large or unsigned enum type is to
request it explicitly with the C++11 strong enum types feature.

However, since LLVM isn't C++11 yet, I had to add a conditional
LLVM_ENUM_INT_TYPE to Compiler.h to control its usage.

The motivating true positive for this change is compiling PointerIntPair
with MSVC for win64.  The PointerIntMask value is supposed to be pointer
sized value of all ones with some low zeros.  Instead, it's truncated to
32-bits!  We are only saved later because it is sign extended back in
the AND with int64_t, and we happen to want all ones.

This silences lots of -Wmicrosoft warnings during a clang self-host
targeting Windows.