implement a transparent optimization with the diagnostics stuff:

implement a transparent optimization with the diagnostics stuff:
const char*'s are now not converted to std::strings when the diagnostic
is formed, we just hold onto their pointer and format as needed.

This commit makes DiagnosticClient::FormatDiagnostic even more of a
mess, I'll fix it in the next commit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@59593 91177308-0d34-0410-b5e6-96231b3b80d8