COFF: add support for CONSTANT exports

The CONSTANT export type is marked as obsolete, but link still supports
this.  Furthermore, WinObjC uses this for certain exports.  Add support
for this export type.

llvm-svn: 301013

lld/COFF/ModuleDef.cpp

@@ -38,6 +38,7 @@ enum Kind {
   Comma,
   Equal,
   KwBase,
+  KwConstant,
   KwData,
   KwExports,
   KwHeapsize,
@@ -92,6 +93,7 @@ public:
       StringRef Word = Buf.substr(0, End);
       Kind K = llvm::StringSwitch<Kind>(Word)
                    .Case("BASE", KwBase)
+                   .Case("CONSTANT", KwConstant)
                    .Case("DATA", KwData)
                    .Case("EXPORTS", KwExports)
                    .Case("HEAPSIZE", KwHeapsize)
@@ -227,6 +229,11 @@ private:
         E.Data = true;
         continue;
       }
+      if (Tok.K == KwConstant) {
+        warn("CONSTANT keyword is obsolete; use DATA");
+        E.Constant = true;
+        continue;
+      }
       if (Tok.K == KwPrivate) {
         E.Private = true;
         continue;