[CodeCompletion] Don't try to resolve generic env for extension in inactive block

Fixes code-completion crash in validateExtension().

When code completion happens, it tries to validate decl contexts. If
it's in an extension in inactivec conditional block, the extension
haven't been bound to nominal type. That used to cause crash because its
GenericParams hasn't been set.

rdar://problem/45275782
https://bugs.swift.org/browse/SR-9001

Author: rintaro

lib/Sema/TypeCheckDecl.cpp

@@ -4970,6 +4970,13 @@ void TypeChecker::validateExtension(ExtensionDecl *ext) {
   if (!isa<SourceFile>(dc))
     return;
 
+  // If this is not bound to any decls at this point, this extension is in
+  // inactive coditional compilation block. It's not safe to typecheck this
+  // extension. This happens if code completion is triggered in inactive
+  // conditional complation block.
+  if (!ext->alreadyBoundToNominal())
+    return;
+
   // Validate the nominal type declaration being extended.
   validateDecl(nominal);
 

validation-test/IDE/crashers_2_fixed/sr9001.swift

@@ -0,0 +1,20 @@
+// RUN: %target-swift-ide-test -code-completion -code-completion-token=COMPLETE_1 -source-filename=%s
+// RUN: %target-swift-ide-test -code-completion -code-completion-token=COMPLETE_2 -source-filename=%s
+
+struct MyStruct<T> {
+}
+
+protocol MyProto {
+  associatedtype Assoc
+  func foo(x: Assoc) -> Assoc
+}
+
+#if false
+extension MyStruct {
+  #^COMPLETE_1^#
+}
+
+extension MyStruct: MyProto {
+  #^COMPLETE_2^#
+}
+#endif