Move diagnostic about @_weakLinked not working for COFF into the type checker

This allows us to use `@_weakLinked` inside of a `#if` properly.

Author: DougGregor

lib/Parse/ParseDecl.cpp

@@ -2873,14 +2873,6 @@ ParserStatus Parser::parseNewDeclAttribute(DeclAttributes &Attributes,
     DiscardAttribute = true;
   }
 
-  if (Context.LangOpts.Target.isOSBinFormatCOFF()) {
-    if (DK == DeclAttrKind::WeakLinked) {
-      diagnose(Loc, diag::attr_unsupported_on_target, AttrName,
-               Context.LangOpts.Target.str());
-      DiscardAttribute = true;
-    }
-  }
-
   if (DK == DeclAttrKind::ResultDependsOnSelf &&
       !Context.LangOpts.hasFeature(Feature::NonescapableTypes)) {
     diagnose(Loc, diag::requires_experimental_feature, AttrName, true,

lib/Sema/TypeCheckAttr.cpp

@@ -147,7 +147,6 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
   IGNORED_ATTR(StaticInitializeObjCMetadata)
   IGNORED_ATTR(SynthesizedProtocol)
   IGNORED_ATTR(Testable)
-  IGNORED_ATTR(WeakLinked)
   IGNORED_ATTR(PrivateImport)
   IGNORED_ATTR(DisfavoredOverload)
   IGNORED_ATTR(ProjectedValueProperty)
@@ -368,6 +367,7 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
   void visitUnsafeNonEscapableResultAttr(UnsafeNonEscapableResultAttr *attr);
 
   void visitStaticExclusiveOnlyAttr(StaticExclusiveOnlyAttr *attr);
+  void visitWeakLinkedAttr(WeakLinkedAttr *attr);
 };
 
 } // end anonymous namespace
@@ -7342,6 +7342,13 @@ void AttributeChecker::visitStaticExclusiveOnlyAttr(
   }
 }
 
+void AttributeChecker::visitWeakLinkedAttr(WeakLinkedAttr *attr) {
+  if (Ctx.LangOpts.Target.isOSBinFormatCOFF()) {
+    diagnoseAndRemoveAttr(attr, diag::attr_unsupported_on_target, AttrName,
+                          Context.LangOpts.Target.str());
+  }
+}
+
 namespace {
 
 class ClosureAttributeChecker

test/attr/attr_weaklinked.swift

@@ -0,0 +1,6 @@
+// RUN: %target-typecheck-verify-swift
+
+#if !os(Windows)
+@_weakLinked public func f() { }
+#endif
+