Make COFF linker work when it's built by clang again.

clang-cl doesn't compile std::atomic_flag correctly (PR24101).  Since the COFF
linker doesn't use threads yet, just revert r241420 and r241481 for now to
work around this clang-cl bug.

llvm-svn: 242006
diff --git a/lld/COFF/InputFiles.cpp b/lld/COFF/InputFiles.cpp
index 66d792b..be5c0ed 100644
--- a/lld/COFF/InputFiles.cpp
+++ b/lld/COFF/InputFiles.cpp
@@ -71,12 +71,6 @@
     if (B->getName() != "__NULL_IMPORT_DESCRIPTOR")
       LazySymbols.push_back(B);
   }
-
-  // Seen is a map from member files to boolean values. Initially
-  // all members are mapped to false, which indicates all these files
-  // are not read yet.
-  for (const Archive::Child &Child : File->children())
-    Seen[Child.getBuffer().data()].clear();
   return std::error_code();
 }
 
@@ -90,7 +84,8 @@
 
   // Return an empty buffer if we have already returned the same buffer.
   const char *StartAddr = It->getBuffer().data();
-  if (Seen[StartAddr].test_and_set())
+  auto Pair = Seen.insert(StartAddr);
+  if (!Pair.second)
     return MemoryBufferRef();
   return It->getMemoryBufferRef();
 }