[clang-rename] Prune away AST nodes more correctly and effectively when looking for a point

Due to the way the preprocessor works nodes can be half in a macro or a
different file. This means checking the file name of the start location
of a Decl is not a correct way of checking if the entire Decl is in that
file. Remove that flawed assumption and replace it with a more effective
check: If the point we're looking for is *inside* of the begin and end
location of a Decl, look inside.

This should make clang-rename more reliable (for example macro'd namespaces
threw it off before, as seen in the test case) and maybe a little faster
by pruning off more stuff that the RecursiveASTVisitor doesn't have to
drill into.

llvm-svn: 287649