[Bug 25404] Fix crash on typedef in OpenCL 2.0

Fixed the assertion due to absence of source location for
implicitly defined types (using addImplicitTypedef()).
During Sema checks the source location is being expected
and therefore an assertion is triggered.

The change is not specific to OpenCL. But it is particularly
common for OpenCL types to be declared implicitly in Clang
to support the mode without the standard header.

Differential Revision: https://reviews.llvm.org/D31397

llvm-svn: 299447