Draft: generic selector thunk #17991

Implements a generic selector thunk which can select any field in a
record. Instead of hardcoding the field offset in the selector code it
is stored in the heap object.

This also adds a filecheck_compile testtype to the testsuite driver
which lets you put "CHECK: something" lines in the source file and check
that the compiler output has lines mentioning the "something" in order.
This is inspired by LLVM's FileCheck[1]

[1]: https://llvm.org/docs/CommandGuide/FileCheck.html