[clang][Basic] Make SourceLocation usable as key in hash maps, NFCI

This change creates a `DenseMapInfo` trait specialization for the
SourceLocation class. The empty key, the tombstone key, and the hash
function are identical to `DenseMapInfo<unsigned>`, because we already
have hash maps that use raw the representation of `SourceLocation` as
a key.

The update of existing `DenseMap`s containing raw representation of
`SourceLocation`s will be done in a follow-up patch. As an example
the patch makes use of the new trait in one instance:
clang-tidy/google/UpgradeGoogletestCaseCheck.{h,cpp}

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D89719