[clang] Use SourceLocation as key in hash maps, NFCI

The patch adjusts the existing `llvm::DenseMap<unsigned, T>` and
`llvm::DenseSet<unsigned>` objects that store source locations, so
that they use `SourceLocation` directly instead of `unsigned`.

This patch relies on the `DenseMapInfo` trait added in D89719.

It also replaces the construction of `SourceLocation` objects from
the constants -1 and -2 with calls to the trait's methods `getEmptyKey`
and `getTombstoneKey` where appropriate.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D69840