[ASTMatchers] extract public matchers from const-analysis into own patch
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.
Reviewed By: aaron.ballman
Differential Revision: https://reviews.llvm.org/D72505