[clang] Remove the DIFlagArgumentNotModified debug info flag

It turns out that the ExprMutationAnalyzer can be very slow when AST
gets huge in some cases. The idea is to move this analysis to the LLVM
back-end level (more precisely, in the LiveDebugValues pass). The new
approach will remove the performance regression, simplify the
implementation and give us front-end independent implementation.

Differential Revision: https://reviews.llvm.org/D68206