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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>