Fix assertion failure in TransformOpaqueValueExpr

Summary:
`OpaqueValueExpr`s may not have a source expression (as in the case when
they are created due to a default argument error).
This can cause an assertion failure in `TransformOpaqueValueExpr` during
template instantiation.

This patch fixes the assertion failure.

Reviewers: hfinkel, rsmith

Subscribers: fraggamuffin, cfe-commits

Differential Revision: http://reviews.llvm.org/D11582

Patch by Rachel Craik!

llvm-svn: 246600