Change default traversal in AST Matchers to ignore invisible nodes

This makes many scenarios simpler by not requiring the user to write
ignoringImplicit() all the time, nor to account for non-visible
cxxConstructExpr() and cxxMemberCalExpr() nodes. This is also, in part,
inclusive of the equivalent of adding a use of ignoringParenImpCasts()
between all expr()-related matchers in an expression.

The pre-existing traverse(TK_AsIs, ...) matcher can be used to explcitly
match on implicit/invisible nodes. See

  http://lists.llvm.org/pipermail/cfe-dev/2019-December/064143.html

for more

Reviewers: aaron.ballman

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72534