Fix new tuple deforming code so it can support cstrings again

Fix new tuple deforming code so it can support cstrings again

In c456e3911, I mistakenly thought that the deformer code would never
see cstrings and that I could use pg_assume() to have the compiler omit
producing code for attlen == -2 attributes.  That saves bloating the
deforming code a bit with the extra check and strlen() call.  While this
is ok to do for tuples from the heap, it's not ok to do for
MinimalTuples as those *can* contain cstrings and
tts_minimal_getsomeattrs() implements deforming by inlining the
(slightly misleadingly named) slot_deform_heap_tuple() code.

To fix, add a new parameter to the slot_deform_heap_tuple() and have the
callers define which code to inline.  Because this new parameter is
passed as a const, the compiler can choose to emit or not emit the
cstring-related code based on the parameter's value.

Author: David Rowley <dgrowleyml@gmail.com>
Reported-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/CAHewXNmSK+gKziAt_WvQoMVWt3_LRVMmRYY9dAbMPMcpPV0QmA@mail.gmail.com