TClingUtils: Avoid growing paths in GetFileName

Strip of ../ so that the search finds a valid longest match. With the
upcoming upgrade of LLVM 13, this would otherwise lead to an error
"File name too long" but it's already wrong now.

core/clingutils/src/TClingUtils.cxx

@@ -3376,7 +3376,11 @@ llvm::StringRef ROOT::TMetaUtils::GetFileName(const clang::Decl& decl,
    }
 
    if (!headerFE) return invalidFilename;
-   llvm::StringRef headerFileName = headerFE->getName();
+
+   llvm::SmallString<256> headerFileName(headerFE->getName());
+   // Remove double ../ from the path so that the search below finds a valid
+   // longest match and does not result in growing paths.
+   llvm::sys::path::remove_dots(headerFileName, /*remove_dot_dot=*/true);
 
    // Now headerFID references the last valid system header or the original
    // user file.