Fix FileCheck --check-prefix lines.

Various tests had sprung up over the years which had --check-prefix=ABC on the
RUN line, but "CHECK-ABC:" later on. This happened to work before, but was
strictly incorrect. FileCheck is getting stricter soon though.

Patch by Ron Ofir.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188174 91177308-0d34-0410-b5e6-96231b3b80d8