[NFC] Make DeclContext::noload_lookup() accept transparent context

Now the `DeclContext::noload_lookup()` asserts that 'this' is not a
transparent context. However, this is not consistent with
`DeclContext::lookup()`, which will lookup into its parent context if
'this' is a transparent context.

This patch makes the behavior of `DeclContext::noload_lookup()` to be
consistent with `DeclContext::lookup()`, to lookup into the parent
context if 'this' is a transparent context.

Author: ChuanqiXu9

clang/lib/AST/DeclBase.cpp

@@ -1852,9 +1852,9 @@ DeclContext::lookup(DeclarationName Name) const {
 
 DeclContext::lookup_result
 DeclContext::noload_lookup(DeclarationName Name) {
-  assert(getDeclKind() != Decl::LinkageSpec &&
-         getDeclKind() != Decl::Export &&
-         "should not perform lookups into transparent contexts");
+  // For transparent DeclContext, we should lookup in their enclosing context.
+  if (getDeclKind() == Decl::LinkageSpec || getDeclKind() == Decl::Export)
+    return getParent()->noload_lookup(Name);
 
   DeclContext *PrimaryContext = getPrimaryContext();
   if (PrimaryContext != this)