Sema: Simpler and faster handleSILGenericParams()

If we're given multiple parameter lists, we don't have to build a
generic signature and environment for each one. Instead, we can
just use a single GenericSignatureBuilder and add all of them in
one shot.

On my iMac Pro, this speeds up the parse_stdlib test from 169
seconds to 133.

Author: slavapestov

lib/Sema/TypeCheckDecl.cpp

@@ -634,6 +634,8 @@ static void setBoundVarsTypeError(Pattern *pattern, ASTContext &ctx) {
 GenericEnvironment *
 TypeChecker::handleSILGenericParams(GenericParamList *genericParams,
                                     DeclContext *DC) {
+  if (genericParams == nullptr)
+    return nullptr;
 
   SmallVector<GenericParamList *, 2> nestedList;
   for (; genericParams; genericParams = genericParams->getOuterParameters()) {
@@ -642,22 +644,15 @@ TypeChecker::handleSILGenericParams(GenericParamList *genericParams,
 
   std::reverse(nestedList.begin(), nestedList.end());
 
-  // Since the innermost GenericParamList is in the beginning of the vector,
-  // we process in reverse order to handle the outermost list first.
-  GenericSignature *parentSig = nullptr;
-  GenericEnvironment *parentEnv = nullptr;
-
   for (unsigned i = 0, e = nestedList.size(); i < e; ++i) {
     auto genericParams = nestedList[i];
     genericParams->setDepth(i);
-
-    parentEnv = checkGenericEnvironment(genericParams, DC, parentSig,
-                                        /*allowConcreteGenericParams=*/true,
-                                        /*ext=*/nullptr);
-    parentSig = parentEnv->getGenericSignature();
   }
 
-  return parentEnv;
+  return checkGenericEnvironment(nestedList.back(), DC,
+                                 /*parentSig=*/nullptr,
+                                 /*allowConcreteGenericParams=*/true,
+                                 /*ext=*/nullptr);
 }
 
 /// Build a default initializer for the given type.