[concurrency] Allow for nonisolated and execution(concurrent) to be used together.

After some discussions, we decided to allow for this since it doesnt harm anything. execution(caller) just takes primacy over it.

Author: gottesmm

lib/Sema/TypeCheckAttr.cpp

@@ -265,16 +265,6 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
 
     switch (attr->getBehavior()) {
     case ExecutionKind::Concurrent: {
-      // 'concurrent' doesn't work with explicit `nonisolated`
-      if (F->hasExplicitIsolationAttribute()) {
-        if (F->getAttrs().hasAttribute<NonisolatedAttr>()) {
-          diagnoseAndRemoveAttr(
-              attr,
-              diag::attr_execution_concurrent_incompatible_with_nonisolated, F);
-          return;
-        }
-      }
-
       auto parameters = F->getParameters();
       if (!parameters)
         return;

test/attr/attr_execution.swift

@@ -42,7 +42,6 @@ do {
 
 struct TestAttributeCollisions {
   @execution(concurrent) nonisolated func testNonIsolated() async {}
-  // expected-error@-1 {{cannot use '@execution(concurrent)' and 'nonisolated' on the same 'testNonIsolated()' because they serve the same purpose}}
 
   @execution(concurrent) func test(arg: isolated MainActor) async {}
   // expected-error@-1 {{cannot use '@execution(concurrent)' on instance method 'test(arg:)' because it has an isolated parameter: 'arg'}}