Allow functions marked @LLDBDebuggerFunction to bypass actor isolation checks

In order to allow LLDB to evaluate expressions inside an actor without spawining
async functions and potentially continue all threads, this relaxes the actor
isolation checks in @LLDBDebuggerFunction functions to allow a synchronous call
into an extension method.

rdar://75905336

Author: adrian-prantl

lib/Sema/TypeCheckConcurrency.cpp

@@ -2391,6 +2391,10 @@ void swift::checkTopLevelActorIsolation(TopLevelCodeDecl *decl) {
 }
 
 void swift::checkFunctionActorIsolation(AbstractFunctionDecl *decl) {
+  // Disable this check for @LLDBDebuggerFunction functions.
+  if (decl->getAttrs().hasAttribute<LLDBDebuggerFunctionAttr>())
+    return;
+
   ActorIsolationChecker checker(decl);
   if (auto body = decl->getBody()) {
     body->walk(checker);

test/Concurrency/LLDBDebuggerFunctionActorExtension.swift

@@ -0,0 +1,21 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-concurrency -debugger-support
+// REQUIRES: concurrency
+
+// This test simulates LLDB's expression evaluator makeing an otherwise illegal
+// synchronous call into an extension of an actor, as it would to run `p n` in
+// this example.
+
+actor A {
+  var n : Int = 0
+}
+
+extension A {
+  final func lldb_wrapped_expr() {
+    n = 1
+  }
+}
+
+@LLDBDebuggerFunction
+func lldb_expr(a: A) {
+  a.lldb_wrapped_expr()
+}