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
Patch originally by Mikhail Maltsev.