RFR: 8248655: Support supplementary characters in String case insensitive operations

[naoto.sato at oracle.com]

Hello,

Please review the fix to the following issues:

https://bugs.openjdk.java.net/browse/JDK-8248655
https://bugs.openjdk.java.net/browse/JDK-8248434

The proposed changeset and its CSR are located at:

https://cr.openjdk.java.net/~naoto/8248655.8248434/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8248664

A bug was filed against SimpleDateFormat (8248434) where 
case-insensitive date format/parse failed in some of the new locales in 
JDK15. The root cause was that case-insensitive String.regionMatches() 
method did not work with supplementary characters. The problem is that 
the method's spec does not expect case mappings of supplementary 
characters, possibly because it was overlooked in the first place, JSR 
204 - "Unicode Supplementary Character support". Similar behavior is 
observed in other two case-insensitive methods, i.e., 
compareToIgnoreCase() and equalsIgnoreCase().

The fix is straightforward to compare strings by code point basis, 
instead of code unit (16bit "char") basis. Technically this change will 
introduce a backward incompatibility, but I believe it is an 
incompatibility to wrong behavior, not true to the meaning of those 
methods' expectations.

Naoto