pgsql: Support plpgsql variable names that conflict with unreserved SQL

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4879a5172af01dc05b6350dfa97ea1ac9a4c603c

Modified Files
--------------
src/pl/plpgsql/src/pl_comp.c | 13 +++---
src/pl/plpgsql/src/pl_scanner.c | 74 +++++++++++++++--------------------
src/pl/plpgsql/src/plpgsql.h | 2 +-
src/test/regress/expected/plpgsql.out | 21 ++++++++++
src/test/regress/sql/plpgsql.sql | 14 +++++++
5 files changed, 75 insertions(+), 49 deletions(-)