[clang] Use SourceLocation as key in std::map, NFCI

SourceLocation implements `operator<`, so `SourceLocation`-s can be used
as keys in `std::map` directly, there is no need to extract the internal
representation.

Since the `operator<` simply compares the internal representations of
its operands, this patch does not introduce any functional changes.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D89705