Fix unintentional behavior change in commit e9931bfb75.

Prior to that commit, there was special case to use ASCII case mapping
behavior for the libc provider with a single-byte encoding when that's
the default collation. Commit e9931bfb75 mistakenly eliminated that
special case; this commit restores it.

Discussion: https://postgr.es/m/01a104f0d2179d756261e90d96fd65c36ad6fcf0.camel@j-davis.com

src/backend/utils/adt/formatting.c

@@ -1755,7 +1755,12 @@ str_tolower(const char *buff, size_t nbytes, Oid collid)
 				 * collations you get exactly what the collation says.
 				 */
 				for (p = result; *p; p++)
-					*p = tolower_l((unsigned char) *p, mylocale->info.lt);
+				{
+					if (mylocale->is_default)
+						*p = pg_tolower((unsigned char) *p);
+					else
+						*p = tolower_l((unsigned char) *p, mylocale->info.lt);
+				}
 			}
 		}
 	}
@@ -1892,7 +1897,12 @@ str_toupper(const char *buff, size_t nbytes, Oid collid)
 				 * collations you get exactly what the collation says.
 				 */
 				for (p = result; *p; p++)
-					*p = toupper_l((unsigned char) *p, mylocale->info.lt);
+				{
+					if (mylocale->is_default)
+						*p = pg_toupper((unsigned char) *p);
+					else
+						*p = toupper_l((unsigned char) *p, mylocale->info.lt);
+				}
 			}
 		}
 	}
@@ -2090,10 +2100,20 @@ str_initcap(const char *buff, size_t nbytes, Oid collid)
 				 */
 				for (p = result; *p; p++)
 				{
-					if (wasalnum)
+					if (mylocale->is_default)
-						*p = tolower_l((unsigned char) *p, mylocale->info.lt);
+					{
+						if (wasalnum)
+							*p = pg_tolower((unsigned char) *p);
+						else
+							*p = pg_toupper((unsigned char) *p);
+					}
 					else
-						*p = toupper_l((unsigned char) *p, mylocale->info.lt);
+					{
+						if (wasalnum)
+							*p = tolower_l((unsigned char) *p, mylocale->info.lt);
+						else
+							*p = toupper_l((unsigned char) *p, mylocale->info.lt);
+					}
 					wasalnum = isalnum_l((unsigned char) *p, mylocale->info.lt);
 				}
 			}

src/backend/utils/adt/like.c

@@ -95,6 +95,8 @@ SB_lower_char(unsigned char c, pg_locale_t locale)
 {
 	if (locale->ctype_is_c)
 		return pg_ascii_tolower(c);
+	else if (locale->is_default)
+		return pg_tolower(c);
 	else
 		return tolower_l(c, locale->info.lt);
 }

src/backend/utils/adt/pg_locale.c

@@ -1216,6 +1216,7 @@ create_pg_locale(Oid collid, MemoryContext context)
 
 	result->provider = collform->collprovider;
 	result->deterministic = collform->collisdeterministic;
+	result->is_default = false;
 
 	if (collform->collprovider == COLLPROVIDER_BUILTIN)
 	{
@@ -1409,6 +1410,7 @@ init_database_collation(void)
 
 
 	default_locale.provider = dbform->datlocprovider;
+	default_locale.is_default = true;
 
 	/*
 	 * Default locale is currently always deterministic.  Nondeterministic

src/include/utils/pg_locale.h

@@ -82,6 +82,7 @@ struct pg_locale_struct
 	bool		deterministic;
 	bool		collate_is_c;
 	bool		ctype_is_c;
+	bool		is_default;
 	union
 	{
 		struct