[clang][Lex] Header map search case insensitivity

[troyj]

Summary

Correct D135801 to be case insensitive.

[bruno]

LGTM with minor nitpick: can you reflect in the title that this is related to header maps?

clang/lib/Lex/HeaderSearch.cpp

for (; i != SearchDirs.size(); ++i) {
 
     // We're concerned with only the initial contiguous run of header
     // maps within SearchDirs, which can be 99% of SearchDirs when
     // SearchDirs.size() is ~10000.
     if (!Dir.isHeaderMap())
       break;
 
     // Give earlier keys precedence over identical later keys.
-    auto Callback = [&](StringRef Filename) { Index.try_emplace(Filename, i); };
+    auto Callback = [&](StringRef Filename) {
+      Index.try_emplace(Filename.lower(), i);
+    };
     Dir.getHeaderMap()->forEachKey(Callback);
   }
 
   SearchDirHeaderMapIndex = std::move(Index);
   FirstNonHeaderMapSearchDirIdx = i;
 }
 
 //===----------------------------------------------------------------------===//
if (CacheLookup.StartIt == NextIt) {
       // our search start.  We will fill in our found location below, so prime
       // the start point value.
       CacheLookup.reset(/*NewStartIt=*/NextIt);
 
       if (It == search_dir_begin() && FirstNonHeaderMapSearchDirIdx > 0) {
         // Handle cold misses of user includes in the presence of many header
         // maps.  We avoid searching perhaps thousands of header maps by
         // jumping directly to the correct one or jumping beyond all of them.
-        auto Iter = SearchDirHeaderMapIndex.find(Filename);
+        auto Iter = SearchDirHeaderMapIndex.find(Filename.lower());
         if (Iter == SearchDirHeaderMapIndex.end())
           // Not in index => Skip to first SearchDir after initial header maps
           It = search_dir_nth(FirstNonHeaderMapSearchDirIdx);
         else
           // In index => Start with a specific header map
           It = search_dir_nth(Iter->second);
       }
     }