Added "HTMLDiagnostic", a generic DiagnosticClient (that also implements PathDiagnostic)

so that all diagnostics can be piped to HTML files instead of as text diagnostics using --html-diags.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@48865 91177308-0d34-0410-b5e6-96231b3b80d8