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.cpp

@@ -27,6 +27,8 @@
 
 #include <projectexplorer/projectexplorerconstants.h>
 
+#include <QDir>
+
 namespace CppTools {
 
 CompilerOptionsBuilder::CompilerOptionsBuilder(const ProjectPart &projectPart)
@@ -98,7 +100,7 @@ void CompilerOptionsBuilder::enableExceptions()
     add(QLatin1String("-fexceptions"));
 }
 
-void CompilerOptionsBuilder::addHeaderPathOptions()
+void CompilerOptionsBuilder::addHeaderPathOptions(bool addAsNativePath)
 {
     typedef ProjectPartHeaderPath HeaderPath;
     const QString defaultPrefix = includeOption();
@@ -124,7 +126,10 @@ void CompilerOptionsBuilder::addHeaderPathOptions()
             break;
         }
 
-        result.append(prefix + headerPath.path);
+        QString path = prefix + headerPath.path;
+        path = addAsNativePath ? QDir::toNativeSeparators(path) : path;
+
+        result.append(path);
     }
 
     m_options.append(result);