Adapt to clang::PPCallbacks::InclusionDirective change in Clang 15 trunk

<https://github.com/llvm/llvm-project/commit/d79ad2f1dbc2db63121620f55d6cfa915f2733ac>
"[clang][lex] NFCI: Use FileEntryRef in PPCallbacks::InclusionDirective()"

Change-Id: Ib2ea325cca3be03d3b5dfb9dfc3d870be659b129
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/133085
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

compilerplugins/clang/includeform.cxx

@@ -28,13 +28,18 @@ private:
 
     void InclusionDirective(
         SourceLocation HashLoc, Token const & IncludeTok, StringRef,
-        bool IsAngled, CharSourceRange FilenameRange, FileEntry const * File,
+        bool IsAngled, CharSourceRange FilenameRange,
+#if CLANG_VERSION >= 150000
+        Optional<FileEntryRef> File,
+#else
+        FileEntry const * File,
+#endif
         StringRef SearchPath, StringRef, clang::Module const *, SrcMgr::CharacteristicKind) override
     {
         if (ignoreLocation(HashLoc)) {
             return;
         }
-        if (File == nullptr) { // in case of "fatal error: '...' file not found"
+        if (!File) { // in case of "fatal error: '...' file not found"
             return;
         }
         if (IncludeTok.getIdentifierInfo()->getPPKeywordID() != tok::pp_include)