[clang-tidy] FunctionSizeCheck: wrap FunctionASTVisitor into anon namespace, NFC

This check is relatively simple, and is often being used
as an example. I'm aware of at least two cases, when
simply copying the FunctionASTVisitor class to a new
check resulted in a rather unobvious segfault. Having it
in anonymous namespace prevents such a problem.

No functionality change, so i opted to avoid phabricator,
especially since clang-tidy reviews are seriously jammed.

llvm-svn: 312912