[clang][ObjC] Fix incorrect return type inference for discarded blocks (llvm#154109)

When parsing a block expression we were not entering a new eval context
and as a result when parsing the block body we continue to treat any
return statements as discarded so infer a `void` result.

This fixes the problem by introducing an evaluation context around the
parsing of the body.

Author: ojhunt

clang/lib/Parse/ParseExpr.cpp

@@ -3342,7 +3342,8 @@ ExprResult Parser::ParseBlockLiteralExpression() {
     Actions.ActOnBlockError(CaretLoc, getCurScope());
     return ExprError();
   }
-
+  EnterExpressionEvaluationContextForFunction PotentiallyEvaluated(
+       Actions, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
   StmtResult Stmt(ParseCompoundStatementBody());
   BlockScope.Exit();
   if (!Stmt.isInvalid())

clang/test/SemaObjCXX/discarded-block-type-inference.mm

@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -fobjc-arc -fblocks %s
+
+void  block_receiver(int (^)() );
+
+int f1() {
+  if constexpr (0)
+    (block_receiver)(^{ return 2; });
+  return 1;
+}
+
+int f2() {
+  if constexpr (0)
+    return (^{ return 2; })();
+  return 1;
+}