Clang: Pass on file paths with native separators

libclang 3.8 seems to be sensitive to file paths separators [1]. On Windows,
this led to not updated document annotations and/or crashes after reparsing.

When passing file paths to libclang, convert to native separators.
When getting file paths from libclang, convert back.

This handles:
 * main file path
 * file paths of the unsaved files
 * -I<DIR> arguments, the resource path (for builtins) and the paths to the
   wrapped qt headers
 * included header files from libclang
 * source locations from libclang

Also, minimize the conversion in SourceLocation to a minimum by making
filePath() lazy.

[1] https://llvm.org/bugs/show_bug.cgi?id=28381

Change-Id: If5866f34a6fdc6b34b16c022d3988e8e6eae2a0a
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/cpptools/compileroptionsbuilder.h

@@ -46,7 +46,7 @@ public:
     // Add options based on project part
     virtual void addTargetTriple();
     virtual void enableExceptions();
-    void addHeaderPathOptions();
+    void addHeaderPathOptions(bool addAsNativePath = false);
     void addToolchainAndProjectDefines();
     virtual void addLanguageOption(ProjectFile::Kind fileKind);
     virtual void addOptionsForLanguage(bool checkForBorlandExtensions = true);