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!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246600 91177308-0d34-0410-b5e6-96231b3b80d8