SIL: Fix undefined behavior with method call on nullptr

This happened to work with a null genericEnv before, because we didn't
touch `this` when the input type didn't contain any type parameters.

Author: slavapestov

lib/SIL/IR/SILFunctionType.cpp

@@ -3011,14 +3011,16 @@ CanSILFunctionType swift::buildSILFunctionThunkType(
 
   auto substFormalTypeIntoThunkContext =
       [&](CanType t) -> CanType {
-    return genericEnv->mapTypeIntoContext(
+    return GenericEnvironment::mapTypeIntoContext(
+        genericEnv,
         mapLocalArchetypesOutOfContext(t, baseGenericSig, capturedEnvs))
                ->getCanonicalType();
   };
   auto substLoweredTypeIntoThunkContext =
       [&](CanSILFunctionType t) -> CanSILFunctionType {
     return cast<SILFunctionType>(
-        genericEnv->mapTypeIntoContext(
+        GenericEnvironment::mapTypeIntoContext(
+          genericEnv,
           mapLocalArchetypesOutOfContext(t, baseGenericSig, capturedEnvs))
               ->getCanonicalType());
   };

lib/SILOptimizer/Utils/Generics.cpp

@@ -1420,7 +1420,8 @@ class FunctionSignaturePartialSpecializer {
       SubstitutionMap::get(
         SpecializedGenericSig,
         [&](SubstitutableType *type) -> Type {
-          return CalleeGenericEnv->mapTypeIntoContext(
+          return GenericEnvironment::mapTypeIntoContext(
+              CalleeGenericEnv,
               SpecializedGenericSig.getReducedType(type));
         },
         LookUpConformanceInModule());