Allow TransformTypos to ignore corrections to a specified VarDecl.

This is needed to prevent a TypoExpr from being corrected to a variable
when the TypoExpr is a subexpression of that variable's initializer.

Also exclude more keywords from the correction candidate pool when the
subsequent token is .* or ->* since keywords like "new" or "return"
aren't valid on the left side of those operators.

Fixes PR23140.

llvm-svn: 236519