[clang] improve accuracy of ExprMutAnalyzer

This patch extracts the ExprMutAnalyzer changes from https://reviews.llvm.org/D54943
into its own revision for simpler review and more atomic changes.

The analysis results are improved. Nested expressions (e.g. conditional
operators) are now detected properly. Some edge cases, especially
template induced imprecisions are improved upon.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D88088