Try to fix bad Twine usage in r206791 with std::string

I'm pretty sure I was referencing destroyed temporaries here.  I'm open
to suggestions on how to write this better.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206804 91177308-0d34-0410-b5e6-96231b3b80d8
[Modules] Remove the only use of the DEBUG(...) macro in the compiler
parts of Clang. I don't really have any opinion about whether using that
macro is good or bad, but its odd that this is the only one, and Eric
seemed happy with just nuking it for now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206806 91177308-0d34-0410-b5e6-96231b3b80d8