Dropped the parentheses for #pragma message and its kin in the -E output generator.

This was a suggestion by Jordan Rose since the documented format for these pragmas is without the parentheses.  At the same time, I've increased test coverage too for the preprocessed output.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179771 91177308-0d34-0410-b5e6-96231b3b80d8