Implement a diagnostics callback for the C interface to Clang, so that

clients can format diagnostics as they wish rather than having to
parse standard error. All of the important parts of the front end's
diagnostics are exposed: text, severity, location, source ranges, and
fix-its. The diagnostics callback is now available with
clang_createTranslationUnitFromSource() and
clang_createTranslationUnit().

As part of this change, CXSourceLocation and CXSourceRange got one
pointer larger, since we need to hold on to the SourceManager and
LangOptions structures in the source location. This is the minimum
amount of information needed for the functions that operate on source
locations and ranges (as implemented now). Previously we held on to
the ASTContext, but the diagnostics callback can end up with source
locations when there is no ASTContext (or preprocessor).

Still to do:
  - Code completion needs to support the diagnostics callback, once we
  have the ability to (de-)serialize diagnostics.
  - Eliminate the "displayDiagnostics" argument to createIndex; we'll
  always pass diagnostics to the callback and let it deal with display.

llvm-svn: 94709