[clang-tidy] Fix llvm-include-order check on Windows.

IncludeDirectives struct used a StringRef that pointed to a stack variable
(SmallString<128> FilenameBuffer from PPDirectives.cpp:1513).

http://reviews.llvm.org/D12632

Patch by Marek Kurdej!

llvm-svn: 246856
diff --git a/clang-tools-extra/clang-tidy/llvm/IncludeOrderCheck.cpp b/clang-tools-extra/clang-tidy/llvm/IncludeOrderCheck.cpp
index b5064da..e6d5bc3 100644
--- a/clang-tools-extra/clang-tidy/llvm/IncludeOrderCheck.cpp
+++ b/clang-tools-extra/clang-tidy/llvm/IncludeOrderCheck.cpp
@@ -33,7 +33,7 @@
   struct IncludeDirective {
     SourceLocation Loc;    ///< '#' location in the include directive
     CharSourceRange Range; ///< SourceRange for the file name
-    StringRef Filename;    ///< Filename as a string
+    std::string Filename;  ///< Filename as a string
     bool IsAngled;         ///< true if this was an include with angle brackets
     bool IsMainModule;     ///< true if this was the first include in a file
   };