[ASTMatchers][NFC] Use move semantics when passing matchers around.

Changing matchers to use non-const members and adding r-value overloads of matcher conversions enables move optimisations.
I don't have performance figures but I can say this knocked 120k from the clang-tidy binary(86k was from the .text section) on a Release with assertions build(x86_64-unknown-linux-gnu).

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D98792