[clang] NFC, make more HeaderSearch methods const.

Author: hokein

clang/include/clang/Lex/HeaderSearch.h

@@ -553,10 +553,10 @@ class HeaderSearch {
   /// macro.
   ///
   /// This routine does not consider the effect of \#import
-  bool isFileMultipleIncludeGuarded(const FileEntry *File);
+  bool isFileMultipleIncludeGuarded(const FileEntry *File) const;
 
   /// Determine whether the given file is known to have ever been \#imported.
-  bool hasFileBeenImported(const FileEntry *File) {
+  bool hasFileBeenImported(const FileEntry *File) const {
     const HeaderFileInfo *FI = getExistingFileInfo(File);
     return FI && FI->isImport;
   }

clang/lib/Lex/HeaderSearch.cpp

@@ -1375,7 +1375,7 @@ HeaderSearch::getExistingFileInfo(const FileEntry *FE,
   return HFI;
 }
 
-bool HeaderSearch::isFileMultipleIncludeGuarded(const FileEntry *File) {
+bool HeaderSearch::isFileMultipleIncludeGuarded(const FileEntry *File) const {
   // Check if we've entered this file and found an include guard or #pragma
   // once. Note that we dor't check for #import, because that's not a property
   // of the file itself.