RFR(XS): 8134484: disallow backquotes as heredoc end marker delimiters

[Michael Haupt]

Dear all,

please review this change.
RFE: https://bugs.openjdk.java.net/browse/JDK-8134484
Webrev: http://cr.openjdk.java.net/~mhaupt/8134484/webrev.00

Thanks,

Michael

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