HeaderSearch: Simplify use of FileEntryRef in HeaderSearch::LookupFile, NFC

[dexonsmith]

Summary

Simplify HeaderSearch::LookupFile. Instead of deconstructing a
FileEntryRef into a name and FileEntry and then rebuilding it later,
use it as is. This helps to unblock making the constructor of
FileEntryRef private to FileManager.

[dexonsmith]

I committed this on Friday as 74910cbbd8d1df824ab1d5e742c50641d0fee907.

clang/lib/Lex/HeaderSearch.cpp

if (llvm::sys::path::is_absolute(Filename)) {
     // Otherwise, just return the file.
     return getFileAndSuggestModule(Filename, IncludeLoc, nullptr,
                                    /*IsSystemHeaderDir*/false,
                                    RequestingModule, SuggestedModule);
   }
 
   // This is the header that MSVC's header search would have found.
   ModuleMap::KnownHeader MSSuggestedModule;
-  const FileEntry *MSFE_FE = nullptr;
-  StringRef MSFE_Name;
+  Optional<FileEntryRef> MSFE;
 
   // Unless disabled, check to see if the file is in the #includer's
   // directory.  This cannot be based on CurDir, because each includer could be
   // a #include of a subdirectory (#include "foo/bar.h") and a subsequent
   // include of "baz.h" should resolve to "whatever/foo/baz.h".
   // This search is not done for <> headers.
   if (!Includers.empty() && !isAngled && !NoCurDirSearch) {
     SmallString<1024> TmpDir;
for (const auto &IncluderAndDir : Includers) {
         }
 
         // Otherwise, we found the path via MSVC header search rules.  If
         // -Wmsvc-include is enabled, we have to keep searching to see if we
         // would've found this header in -I or -isystem directories.
         if (Diags.isIgnored(diag::ext_pp_include_search_ms, IncludeLoc)) {
           return FE;
         } else {
-          MSFE_FE = &FE->getFileEntry();
-          MSFE_Name = FE->getName();
+          MSFE = FE;
           if (SuggestedModule) {
             MSSuggestedModule = *SuggestedModule;
             *SuggestedModule = ModuleMap::KnownHeader();
           }
           break;
         }
       }
       First = false;
     }
   }
 
-  Optional<FileEntryRef> MSFE(MSFE_FE ? FileEntryRef(MSFE_Name, *MSFE_FE)
-                                      : Optional<FileEntryRef>());
-
   CurDir = nullptr;
 
   // If this is a system #include, ignore the user #include locs.
   unsigned i = isAngled ? AngledDirIdx : 0;
 
   // If this is a #include_next request, start searching after the directory the
   // file was found in.
   if (FromDir)