ecpg: refactor to eliminate cast-away-const in find_variable().

ecpg: refactor to eliminate cast-away-const in find_variable().

find_variable() and its subroutines transiently scribble on the
passed-in "name" string, even though we've declared that "const".
The string is in fact temporary, so this is not very harmful,
but it's confusing and will produce compiler warnings with
late-model gcc.  Rearrange the code so that instead of modifying
the given string, we make temporary copies of the parts that we
need separated out.  (I used loc_alloc so that the copies are
short-lived and don't need to be freed explicitly.)

This code is poorly structured and confusing, to the point where
my first attempt to fix it was wrong.  It is also under-tested,
allowing the broken v1 patch to nonetheless pass regression.
I'll restrain myself from rewriting it completely, and just add
some comments and more test cases.

We will probably want to back-patch this once gcc 15.2 becomes
more widespread, but for now just put it in master.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1324889.1764886170@sss.pgh.pa.us