[cxx-interop] Workaround name lookup issues with namespace simd

On Apple platforms, a system module `simd` declares a `namespace simd`
under `#if defined(__cplusplus)`. This namespace defines C++ overlays of
the simd types, but these types are already refined for Swift
separately, so it's not necessary to import this namespace.

This is the same issue previously encountered for the `os` module, work
around it in the same way.

rdar://143007477

Author: porglezomp

lib/ClangImporter/ImportDecl.cpp

@@ -1166,6 +1166,14 @@ namespace {
           decl->getOwningModule() &&
           decl->getOwningModule()->getTopLevelModuleName() == "os")
         return nullptr;
+      // Workaround for simd module declaring `namespace simd` on Darwin,
+      // causing name lookup issues. That namespace declares C++ overlays of
+      // types that are already refined for Swift, so let's not import the
+      // namespace (rdar://143007477).
+      if (decl->getIdentifier() && decl->getName() == "simd" &&
+          decl->getOwningModule() &&
+          decl->getOwningModule()->getTopLevelModuleName() == "simd")
+        return nullptr;
       // If this is a top-level namespace, don't put it in the module we're
       // importing, put it in the "__ObjC" module that is implicitly imported.
       if (!decl->getParent()->isNamespace())

test/Interop/Cxx/objc-correctness/simd_quatf.swift

@@ -0,0 +1,8 @@
+// RUN: %target-swift-frontend -typecheck -verify -I %S/Inputs -cxx-interoperability-mode=default %s
+
+// REQUIRES: objc_interop
+// REQUIRES: VENDOR=apple
+
+import simd
+
+var _: simd.simd_quatf! = nil