[-cxx-abi microsoft] Mangle large integral constants correctly

Testing has revealed that large integral constants (i.e. > INT64_MAX)
are always mangled as-if they are negative, even in places where it
would not make sense for them to be negative (like non-type template
parameters of type unsigned long long).

To address this, we change the way we model number mangling: always
mangle as-if our number is an int64_t.  This should result in correct
results when we have large unsigned numbers.

N.B.  Bizarrely, things that are 32-bit displacements like vbptr offsets
are mangled as-if they are unsigned 32-bit numbers.  This is a pretty
egregious waste of space, it would be a 4x savings if we could mangle it
like a signed 32-bit number.  Instead, we explicitly cast these
displacements to uint32_t and let the mangler proceed.

llvm-svn: 196771