When the template specialization header is missing, set a valid source location for the template keyword when recovering.

Otherwise ClassTemplateSpecializationDecl::getSourceRange() will mistakenly consider itself as an implicit partial specialization
and lead to a crash.

Fixes rdar://14063074

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183325 91177308-0d34-0410-b5e6-96231b3b80d8