[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.