Make MagicWordArray compatible with PCRE 8.34+

In PCRE 8.34, a subpattern's name must not start with a digit; work around
this by replacing digits in the synonym's index with letters. Amazingly,
this part of the name seems to have the sole purpose of ensuring uniqueness;
none of the matching functions actually use it for anything.

Adding a single-letter prefix was considered, though it would risk breaking
extension code that may have used 29- or 30- character magic word IDs.
(PCRE limits subpattern names to 32 characters.) Likewise, moving the magic
word's ID to the front would not work if it were to start with a digit.

Bug: 58640
Change-Id: Ic69f9000addbf18c4747105187e6f13191828fbb