[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
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