[sil] Implement helper function SILType::getFieldType(VarDecl *, SILFunction *) const.

This just calls:

```
SILType::getFieldType(VarDecl *, SILModule &, TypeExpansionContext) const
```

using the state in the SILFunction to handle the SILModule and
TypeExpansionContext parameters.

Just trimming down a larger commit into smaller commits.

Author: gottesmm

include/swift/SIL/SILType.h

@@ -495,6 +495,8 @@ class SILType {
   SILType getFieldType(VarDecl *field, SILModule &M,
                        TypeExpansionContext context) const;
 
+  SILType getFieldType(VarDecl *field, SILFunction *fn) const;
+
   /// Given that this is an enum type, return the lowered type of the
   /// data for the given element.  Applies substitutions as necessary.
   /// The result will have the same value category as the base type.

lib/SIL/IR/SILType.cpp

@@ -320,6 +320,10 @@ SILType SILType::getFieldType(VarDecl *field, SILModule &M,
   return getFieldType(field, M.Types, context);
 }
 
+SILType SILType::getFieldType(VarDecl *field, SILFunction *fn) const {
+  return getFieldType(field, fn->getModule(), fn->getTypeExpansionContext());
+}
+
 SILType SILType::getEnumElementType(EnumElementDecl *elt, TypeConverter &TC,
                                     TypeExpansionContext context) const {
   assert(elt->getDeclContext() == getEnumOrBoundGenericEnum());