Support plpgsql variable names that conflict with unreserved SQL keywords.

Support plpgsql variable names that conflict with unreserved SQL keywords.

A variable name matching a statement-introducing keyword, such as
"comment" or "update", caused parse failures if one tried to write
a statement using that keyword.  Commit bb1b8f69 already addressed
this scenario for the case of variable names matching unreserved
plpgsql keywords, but we didn't think about unreserved core-grammar
keywords.  The same heuristic (viz, it can't be a variable name
unless the next token is assignment or '[') should work fine for
that case too, and as a bonus the code gets shorter and less
duplicative.

Per bug #15555 from Feike Steenbergen.  Since this hasn't been
complained of before, and is easily worked around anyway,
I won't risk a back-patch.

Discussion: https://postgr.es/m/15555-149bbd70ddc7b4b6@postgresql.org