Handle OpaqueValueExprs more intelligently in the TransformTypos tree

Handle OpaqueValueExprs more intelligently in the TransformTypos tree
transform.

Also diagnose typos in the initializer of an invalid C++ declaration.
Both issues were hit using the same line of test code, depending on
whether the code was treated as C or C++.

Fixes PR22092.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@225389 91177308-0d34-0410-b5e6-96231b3b80d8