[clang] Change set type used for SourceLocation.

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

If clang is built for a 32-bit platform and SourceLocation is 64 bits
wide, then a SourceLocation will be larger than a pointer, so it won't
be possible to keep them in a SmallPtrSet any more. Switch to
SmallDenseSet instead.

Patch originally by Mikhail Maltsev.

Differential Revision: https://reviews.llvm.org/D105493