Revert "Fix issue 21648: remove two lines from dsymbolsem.d. I don't know what they do but it fixes the bug. And doesn't break the testsuite, so ¯\_(ツ)_/¯"

This reverts commit 1ed39162b0.

src/dmd/dsymbolsem.d

@@ -5963,6 +5963,8 @@ void templateInstanceSemantic(TemplateInstance tempinst, Scope* sc, Expressions*
     }
 
     tempinst.hasNestedArgs(tempinst.tiargs, tempdecl.isstatic);
+    if (tempinst.errors)
+        goto Lerror;
 
     // Copy the tempdecl namespace (not the scope one)
     tempinst.cppnamespace = tempdecl.cppnamespace;

test/fail_compilation/fail21648.d

@@ -1,16 +0,0 @@
-// REQUIRED_ARGS: -de
-/*
-TEST_OUTPUT:
----
-fail_compilation/fail21648.d(10): Error: undefined identifier `semantics`
----
-*/
-
-template bla() {
-    semantics error;
-}
-
-struct Type {
-    enum e = __traits(compiles, Type.init);
-    static if (bla!()) { }
-}