Clang: Skip parsing function bodies in the preamble

With this, we get

 * Significantly faster reparsing (~ 50% faster here for texteditor.cpp)
 * Less crashes due to less code to handle
 * Reduced memory consumption and size of the preamble on disk due to
   smaller AST

On the downside, we don't get anymore diagnostics for invalid template
instantiations, e.g:

 -- header.h --

  template<class T>
  bool cmp(T t1, T t2)
  { return t1 < t2; } // Function body is not parsed.

 -- main.cpp --

  #include "header.h"
  struct Foo {};
  void example(Foo foo)
  {
      cmp(foo, foo);
  }

The diagnostic "error: invalid operands to binary expression ('Foo' and
'Foo')" is not reported anymore. It will be noticed at build time.

Change-Id: I9b37be349f99af88b4786380ca1d8e6aff39b123
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangbackend/source/clangtranslationunitupdater.cpp

@@ -175,6 +175,10 @@ uint TranslationUnitUpdater::defaultParseOptions()
 {
     return CXTranslationUnit_CacheCompletionResults
          | CXTranslationUnit_PrecompiledPreamble
+#ifdef CINDEX_VERSION_HAS_LIMITSKIPFUNCTIONBODIESTOPREAMBLE_BACKPORTED
+         | CXTranslationUnit_SkipFunctionBodies
+         | CXTranslationUnit_LimitSkipFunctionBodiesToPreamble
+#endif
          | CXTranslationUnit_IncludeBriefCommentsInCodeCompletion
          | CXTranslationUnit_DetailedPreprocessingRecord
          | CXTranslationUnit_KeepGoing;