[Typed throws] Always convert error to destination type in calls

An attempted SILGen optimization caused assertions and SIL verification
errors. Always convert the error appropriately.

Fixes rdar://119214492.

Author: DougGregor

lib/SILGen/SILGenStmt.cpp

@@ -1569,7 +1569,7 @@ void SILGenFunction::emitThrow(SILLocation loc, ManagedValue exnMV,
   // If the thrown error type differs from what the throw destination expects,
   // perform the conversion.
   // FIXME: Can the AST tell us what to do here?
-  if (exnType != destErrorType && !shouldDiscard) {
+  if (exnType != destErrorType) {
     assert(destErrorType == SILType::getExceptionType(getASTContext()));
 
     ProtocolConformanceRef conformances[1] = {

test/SILGen/typed_throws.swift

@@ -166,6 +166,16 @@ func optionalTryDifferent() throws(MyError) {
   try? throwsMyBigError()
 }
 
+func throwsMyBigErrorOrReturnsInt() throws(MyBigError) -> Int { 5 }
+
+func mightThrowAny(arg: Int) throws { }
+
+// CHECK-LABEL: sil hidden [ossa] @$s12typed_throws14forceTryErasedyyF : $@convention(thin) () -> () {
+func forceTryErased() {
+  // CHECK: try_apply {{.*}} @error MyBigError
+  // CHECK: try_apply {{.*}} @error any Error
+  try! mightThrowAny(arg: throwsMyBigErrorOrReturnsInt())
+}
 
 // CHECK-LABEL:      sil_vtable MySubclass {
 // CHECK-NEXT:   #MyClass.init!allocator: <E where E : Error> (MyClass.Type) -> (() throws(E) -> ()) throws(E) -> MyClass : @$s12typed_throws10MySubclassC4bodyACyyxYKXE_txYKcs5ErrorRzlufC [override]