[clang] return type not correctly deduced for discarded lambdas

The early return for lamda expressions with deduced return types
in Sema::ActOnCapScopeReturnStmt meant that we were not actually
perform the required return type deduction for such lambdas when
in a discarded context.

This PR removes that early return allowing the existing return
type deduction steps to be performed.

Fixes #GH153884

Fix developed by and

Co-authored-by: Corentin Jabot <[email protected]>

Author: ojhunt

clang/test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp

@@ -239,5 +239,21 @@ void f2() {
 
 }
 
+namespace GH153884 {
+  bool f1() {
+    auto f = [](auto) { return true; };
+    if constexpr (0)
+      return f(1);
+    return false;
+  }
+  bool f2() {
+    auto f = [](auto x) { if (x) return 1.5; else return "wat"; };
+    // expected-error@-1 {{'auto' in return type deduced as 'const char *' here but deduced as 'double' in earlier return statement}}
+    if constexpr (0)
+      return f(1);
+    // expected-note@-1 {{in instantiation of function template specialization 'GH153884::f2()}}
+    return false;
+  }
+}
 
 #endif