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

This reverts commit c7021ae.

The change triggers a compiler regression for some projects that rely on simd and enable C++ interop.

rdar://143352205

Author: egorzhdan

lib/ClangImporter/ImportDecl.cpp

@@ -1166,14 +1166,6 @@ 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())