RFR: 8074297: substring in XSLT returns wrong character if string contains supplementary chars

[Aleksej Efimov]

Hello,
Please review the fix for XSLT 'substring' function that solves the 
problem when string parameter contains supplementary characters.
The 'test/javax/xml/jaxp/transform/8062923/XslSubstringTest.java' test 
was updated to include the test cases for fail behavior. Fixed function 
utilizes the
codePoint functions to calculate String offsets and length, similar to 
'string-length' XSL function.
Testing results: JTREG and JPRT shows no failures with XML related 
tests. Modified test shows no failures.

With Best Regards,
Aleksej

[1] JBS: https://bugs.openjdk.java.net/browse/JDK-8074297
[2] Webrevs:
     jaxp: http://cr.openjdk.java.net/~aefimov/8074297/9/00/jaxp/
     jdk: http://cr.openjdk.java.net/~aefimov/8074297/9/00/jdk/