* Remove isInSystemHeader() from DiagClient, move it to SourceManager

* Move FormatError() from TextDiagnostic up to DiagClient, remove now  
  empty class TextDiagnostic
* Make DiagClient optional for Diagnostic

This fixes the following problems:

* -html-diags (and probably others) does now output the same set of  
  warnings as console clang does
* nothing crashes if one forgets to call setHeaderSearch() on  
  TextDiagnostic
* some code duplication is removed

llvm-svn: 54620