[clang] Introduce SourceLocation::[U]IntTy typedefs.

This is part of a patch series working towards the ability to make
SourceLocation into a 64-bit type to handle larger translation units.

NFC: this patch introduces typedefs for the integer type used by
SourceLocation and makes all the boring changes to use the typedefs
everywhere, but for the moment, they are unconditionally defined to
uint32_t.

Patch originally by Mikhail Maltsev.

Reviewed By: tmatheson

Differential Revision: https://reviews.llvm.org/D105492