Sema: Fix capture lifetime issue in protocol conformance sendable mismatch

The closure passed in to diagnoseOrDefer() outlives the ConformanceChecker
instance, so instead of capturing 'this', pull out the 'DC' instance
variable directly.

Author: slavapestov

lib/Sema/TypeCheckProtocol.cpp

@@ -4219,8 +4219,10 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
       if (behavior != DiagnosticBehavior::Ignore) {
         bool isError = behavior < DiagnosticBehavior::Warning;
 
+        // Avoid relying on the lifetime of 'this'.
+        const DeclContext *DC = this->DC;
         diagnoseOrDefer(requirement, isError,
-                        [this, requirement, witness, sendFrom](
+                        [DC, requirement, witness, sendFrom](
                           NormalProtocolConformance *conformance) {
           diagnoseSendabilityErrorBasedOn(conformance->getProtocol(), sendFrom,
                                           [&](DiagnosticBehavior limit) {

test/Concurrency/sendable_witness_check_delayed.swift

@@ -0,0 +1,16 @@
+// RUN: %target-typecheck-verify-swift
+
+// This triggers a conformance check with SuppressDiagnostics=true.
+let x = S().f {}
+
+protocol P {
+  associatedtype A
+
+  func f(_: A) -> Int // expected-note {{expected sendability to match requirement here}}
+}
+
+struct S : P {
+  typealias A = () -> ()
+  func f(_: @Sendable () -> ()) -> Int { return 0 }
+  // expected-warning@-1 {{sendability of function types in instance method 'f' does not match requirement in protocol 'P'}}
+}