Rethink the idea of having plpgsql depend on parser/gram.h. Aside from the

fact that this is breaking the MSVC build, it's probably not really a good
idea to expand the dependencies of gram.h any further than the core parser;
for instance the value of SCONST might depend on which bison version you'd
built with.  Better to expose an additional call point in parser.c, so
move what I had put into pl_funcs.c into parser.c.  Also PGDLLIMPORT'ify
the reference to standard_conforming_strings, per buildfarm results.