[SILGen] Compute the reduced thrown error type when emitting a function

prolog and epilog.

(cherry picked from commit 5041986)

Author: hborla

lib/SILGen/SILGenEpilog.cpp

@@ -65,6 +65,7 @@ void SILGenFunction::prepareEpilog(
 
   if (errorType) {
     auto genericSig = DC->getGenericSignatureOfContext();
+    errorType = (*errorType)->getReducedType(genericSig);
     AbstractionPattern origErrorType = TypeContext
       ? *TypeContext->OrigType.getFunctionThrownErrorType()
       : AbstractionPattern(genericSig.getCanonicalSignature(),

lib/SILGen/SILGenProlog.cpp

@@ -1548,6 +1548,8 @@ uint16_t SILGenFunction::emitBasicProlog(
   // Create the indirect result parameters.
   auto genericSig = DC->getGenericSignatureOfContext();
   resultType = resultType->getReducedType(genericSig);
+  if (errorType)
+    errorType = (*errorType)->getReducedType(genericSig);
 
   std::optional<AbstractionPattern> origClosureType;
   if (TypeContext) origClosureType = TypeContext->OrigType;

test/SILGen/typed_throws_generic.swift

@@ -378,3 +378,12 @@ struct GSF2<F: Error, T>: P2 {
 struct GSA<T>: P2 {
   typealias Failure = any Error
 }
+
+struct ReducedError<T: Error> {}
+
+extension ReducedError where T == MyError {
+  // CHECK-LABEL: sil hidden [ossa] @$s20typed_throws_generic12ReducedErrorVA2A02MyE0ORszrlE05throwfE0yyAEYKF : $@convention(method) (ReducedError<MyError>) -> @error MyError {
+  func throwMyError() throws(T) {
+    throw MyError.fail
+  }
+}