Move comment about casts from pg_wchar.

Suggested-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/CA+hUKGLXQUYK7Cq5KbLGgTWo7pORs7yhBWO1AEnZt7xTYbLRhg@mail.gmail.com

src/backend/utils/adt/pg_locale_icu.c

@@ -128,6 +128,11 @@ char_is_cased_icu(char ch, pg_locale_t locale)
 		(ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z');
 }
 
+/*
+ * XXX: many of the functions below rely on casts directly from pg_wchar to
+ * UChar32, which is correct for the UTF-8 encoding, but not in general.
+ */
+
 static pg_wchar
 toupper_icu(pg_wchar wc, pg_locale_t locale)
 {

src/backend/utils/adt/pg_locale_libc.c

@@ -45,8 +45,7 @@
  *
  * 2. When working in UTF8 encoding, we use the <wctype.h> functions.
  * This assumes that every platform uses Unicode codepoints directly
- * as the wchar_t representation of Unicode.  (XXX: ICU makes this assumption
- * even for non-UTF8 encodings, which may be a problem.)  On some platforms
+ * as the wchar_t representation of Unicode.  On some platforms
  * wchar_t is only 16 bits wide, so we have to punt for codepoints > 0xFFFF.
  *
  * 3. In all other encodings, we use the <ctype.h> functions for pg_wchar