Fix 32-bit builds

r303972 used GetValueForKeyAsInteger with mismatched types (e.g.
instantiating with uint64_t, but passing a size_t argument), which
manifested itself on 32-bit architectures.

The intended usage of these functions was to not specify the type
explicitly, and let the compiler figure that out, so switch to that kind
of usage instead.

llvm-svn: 303988