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/tools/clangbackend/ipcsource/clangtranslationunit.cpp

@@ -26,6 +26,7 @@
 #include "clangtranslationunit.h"
 
 #include "cursor.h"
+#include "clangfilepath.h"
 #include "clangstring.h"
 #include "codecompleter.h"
 #include "commandlinearguments.h"
@@ -475,7 +476,8 @@ void TranslationUnit::includeCallback(CXFile included_file,
 
     TranslationUnit *translationUnit = static_cast<TranslationUnit*>(clientData);
 
-    translationUnit->d->dependedFilePaths.insert(includeFilePath);
+    const Utf8String normalizedFilePath = FilePath::fromNativeSeparators(includeFilePath);
+    translationUnit->d->dependedFilePaths.insert(normalizedFilePath);
 }
 
 UnsavedFiles &TranslationUnit::unsavedFiles() const