Revise plpgsql's scanner to process comments and string literals in a way

more nearly matching the core SQL scanner.  The user-visible effects are:

* Block comments (slash-star comments) now nest, as per SQL spec.

* In standard_conforming_strings mode, backslash as the last character of a
  non-E string literal is now correctly taken as an ordinary character;
  formerly it was misinterpreted as escaping the ending quote.  (Since the
  string also had to pass through the core scanner, this invariably led
  to syntax errors.)

* Formerly, backslashes in the format string of RAISE were always treated as
  quoting the next character, regardless of mode.  Now, they are ordinary
  characters with standard_conforming_strings on, while with it off, they
  introduce the same set of escapes as in the core SQL scanner.  Also,
  escape_string_warning is now effective for RAISE format strings.  These
  changes make RAISE format strings work just like any other string literal.

This is implemented by copying and pasting a lot of logic from the core
scanner.  It would be a good idea to look into getting rid of plpgsql's
scanner entirely in favor of using the core scanner.  However, that involves
more change than I can justify making during beta --- in particular, the core
scanner would have to become re-entrant.

In passing, remove the kluge that made the plpgsql scanner emit T_FUNCTION or
T_TRIGGER as a made-up first token.  That presumably had some value once upon
a time, but now it's just useless complication for both the scanner and the
grammar.

src/test/regress/sql/plpgsql.sql

@@ -3005,3 +3005,47 @@ SELECT * FROM leaker_1(true);
 
 DROP FUNCTION leaker_1(bool);
 DROP FUNCTION leaker_2(bool);
+
+-- Test handling of string literals.
+
+set standard_conforming_strings = off;
+
+create or replace function strtest() returns text as $$
+begin
+  raise notice 'foo\\bar\041baz';
+  return 'foo\\bar\041baz';
+end
+$$ language plpgsql;
+
+select strtest();
+
+create or replace function strtest() returns text as $$
+begin
+  raise notice E'foo\\bar\041baz';
+  return E'foo\\bar\041baz';
+end
+$$ language plpgsql;
+
+select strtest();
+
+set standard_conforming_strings = on;
+
+create or replace function strtest() returns text as $$
+begin
+  raise notice 'foo\\bar\041baz\';
+  return 'foo\\bar\041baz\';
+end
+$$ language plpgsql;
+
+select strtest();
+
+create or replace function strtest() returns text as $$
+begin
+  raise notice E'foo\\bar\041baz';
+  return E'foo\\bar\041baz';
+end
+$$ language plpgsql;
+
+select strtest();
+
+drop function strtest();