Sema: Build type checked bodies for trivial accessors

This includes all synthesized accessors except for lazy getters
and observer setters.

Building checked AST has certain advantages:

- It is faster, avoiding the need to create and solve ConstraintSystems
  or performing various diagnostic and semantic walks over the AST.

- It allows us to postpone delayed body synthesis until needed in SILGen,
  instead of having to walk a list of "external declarations" in Sema.

It also unblocks lazier conformance checking. Now that SILGen can
trigger conformance checking on its own, we need to be able to
synthesize bodies of accessors that witness protocol requirements.
This will allow us to eventually remove Sema's "used conformances"
list.

Note that for now, various utility functions in CodeSynthesis.cpp are
used for both checked and unchecked function bodies, so they take a
'typeChecked' boolean parameter that complicates some logic more than
necessary. Once the remaining body synthesizers are refactored to
build type-checked AST, the 'typeChecked' flag will go away.

Author: slavapestov