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/codecompleter.cpp

@@ -25,6 +25,7 @@
 
 #include "codecompleter.h"
 
+#include "clangfilepath.h"
 #include "clangcodecompleteresults.h"
 #include "clangstring.h"
 #include "cursor.h"
@@ -83,8 +84,10 @@ ClangCodeCompleteResults CodeCompleter::complete(uint line,
                                                  CXUnsavedFile *unsavedFiles,
                                                  unsigned unsavedFileCount)
 {
+    const Utf8String nativeFilePath = FilePath::toNativeSeparators(translationUnit.filePath());
+
     return clang_codeCompleteAt(translationUnit.cxTranslationUnitWithoutReparsing(),
-                                translationUnit.filePath().constData(),
+                                nativeFilePath.constData(),
                                 line,
                                 column,
                                 unsavedFiles,