[LLDB][NativePDB] Allow find functions by full names

I don't see a reason why not to. If we allows lookup functions by full names,
I can change the test case in D113930 to use `lldb-test symbols --find=function --name=full::name --function-flags=full ...`,
though the duplicate method decl prolem is still there for `lldb-test symbols --dump-ast`.
That's a seprate bug, we can fix it later.

Differential Revision: https://reviews.llvm.org/D114467