Prevent CHECK-NOTs from matching file paths

A build directory with a name like `build-Werror` would hit a false
positive on these `CHECK-NOT`s before, since the actual error line looks
like:

    .../build-Werror/bin/llvm-as <stdin>:1:2: error: ...

Switch to using:

    CHECK-NOT: error:

(note the trailing semi-colon) to avoid matching almost any file path.

llvm-svn: 232917