Prevent duplicate escape-string warnings when using pg_stat_statements.

contrib/pg_stat_statements will sometimes run the core lexer a second time
on submitted statements.  Formerly, if you had standard_conforming_strings
turned off, this led to sometimes getting two copies of any warnings
enabled by escape_string_warning.  While this is probably no longer a big
deal in the field, it's a pain for regression testing.

To fix, change the lexer so it doesn't consult the escape_string_warning
GUC variable directly, but looks at a copy in the core_yy_extra_type state
struct.  Then, pg_stat_statements can change that copy to disable warnings
while it's redoing the lexing.

It seemed like a good idea to make this happen for all three of the GUCs
consulted by the lexer, not just escape_string_warning.  There's not an
immediate use-case for callers to adjust the other two AFAIK, but making
it possible is easy enough and seems like good future-proofing.

Arguably this is a bug fix, but there doesn't seem to be enough interest to
justify a back-patch.  We'd not be able to back-patch exactly as-is anyway,
for fear of breaking ABI compatibility of the struct.  (We could perhaps
back-patch the addition of only escape_string_warning by adding it at the
end of the struct, where there's currently alignment padding space.)

src/include/parser/scanner.h

@@ -77,6 +77,16 @@ typedef struct core_yy_extra_type
 	const ScanKeyword *keywords;
 	int			num_keywords;
 
+	/*
+	 * Scanner settings to use.  These are initialized from the corresponding
+	 * GUC variables by scanner_init().  Callers can modify them after
+	 * scanner_init() if they don't want the scanner's behavior to follow the
+	 * prevailing GUC settings.
+	 */
+	int			backslash_quote;
+	bool		escape_string_warning;
+	bool		standard_conforming_strings;
+
 	/*
 	 * literalbuf is used to accumulate literal values when multiple rules are
 	 * needed to parse a single literal.  Call startlit() to reset buffer to