Avoid global LC_CTYPE dependency in pg_locale_libc.c.

Call tolower_l() directly instead of through pg_tolower(), because the
latter depends on the global LC_CTYPE.

Discussion: https://postgr.es/m/8186b28a1a39e61a0d833a4c25a8909ebbbabd48.camel@j-davis.com

src/backend/utils/adt/pg_locale_libc.c

@@ -450,7 +450,12 @@ strlower_libc_sb(char *dest, size_t destsize, const char *src, ssize_t srclen,
 		for (p = dest; *p; p++)
 		{
 			if (locale->is_default)
-				*p = pg_tolower((unsigned char) *p);
+			{
+				if (*p >= 'A' && *p <= 'Z')
+					*p += 'a' - 'A';
+				else if (IS_HIGHBIT_SET(*p) && isupper_l(*p, loc))
+					*p = tolower_l((unsigned char) *p, loc);
+			}
 			else
 				*p = tolower_l((unsigned char) *p, loc);
 		}
@@ -535,9 +540,19 @@ strtitle_libc_sb(char *dest, size_t destsize, const char *src, ssize_t srclen,
 			if (locale->is_default)
 			{
 				if (wasalnum)
-					*p = pg_tolower((unsigned char) *p);
+				{
+					if (*p >= 'A' && *p <= 'Z')
+						*p += 'a' - 'A';
+					else if (IS_HIGHBIT_SET(*p) && isupper_l(*p, loc))
+						*p = tolower_l((unsigned char) *p, loc);
+				}
 				else
-					*p = pg_toupper((unsigned char) *p);
+				{
+					if (*p >= 'a' && *p <= 'z')
+						*p -= 'a' - 'A';
+					else if (IS_HIGHBIT_SET(*p) && islower_l(*p, loc))
+						*p = toupper_l((unsigned char) *p, loc);
+				}
 			}
 			else
 			{
@@ -633,7 +648,12 @@ strupper_libc_sb(char *dest, size_t destsize, const char *src, ssize_t srclen,
 		for (p = dest; *p; p++)
 		{
 			if (locale->is_default)
-				*p = pg_toupper((unsigned char) *p);
+			{
+				if (*p >= 'a' && *p <= 'z')
+					*p -= 'a' - 'A';
+				else if (IS_HIGHBIT_SET(*p) && islower_l(*p, loc))
+					*p = toupper_l((unsigned char) *p, loc);
+			}
 			else
 				*p = toupper_l((unsigned char) *p, loc);
 		}