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/commandlinearguments.cpp

@@ -25,8 +25,12 @@
 
 #include "commandlinearguments.h"
 
+#include "clangfilepath.h"
+
 #include <utf8string.h>
 
+#include <QByteArray>
+
 #include <iostream>
 
 namespace ClangBackEnd {
@@ -46,7 +50,8 @@ CommandLineArguments::CommandLineArguments(const char *filePath,
         m_arguments.push_back(argument.constData());
     if (addVerboseOption)
         m_arguments.push_back("-v");
-    m_arguments.push_back(filePath);
+    m_nativeFilePath = FilePath::toNativeSeparators(Utf8String::fromUtf8(filePath));
+    m_arguments.push_back(m_nativeFilePath.constData());
 }
 
 const char * const *CommandLineArguments::data() const