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

* 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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@54620 91177308-0d34-0410-b5e6-96231b3b80d8