[ASTMatchers] extract public matchers from const-analysis into own patch

Summary:
The analysis for const-ness of local variables required a view generally useful
matchers that are extracted into its own patch.

They are `decompositionDecl` and `forEachArgumentWithParamType`, that works
for calls through function pointers as well.

Reviewers: aaron.ballman

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72505