Add missing transaction
Avoid unnecessary lookup and string comparison.

To get the value of TMethodCall::ReturnType rather than doing a fresh
lookup from the string representation of the return type *and* then still
doing string comparison on the partial simplified result,
we know just use the interface of the clang::QualType that is attached
to the underlying function decl.

This also save all the memory cost associated with calling findType.
Avoid unnecessary set up of the parser and memory buffer creation.