Sets or clears the leading comment string. The comment
string must include the starting and ending comment delimiters,
and any embedded linebreaks.
A leading comment is a comment that appears before the statement.
It may be either a traditional comment or an end-of-line comment.
Traditional comments must begin with "/*, may contain line breaks,
and must end with "*/. End-of-line comments must begin with "//"
(as per JLS 3.7), and must not contain line breaks.
Examples:
setLeadingComment("/* traditional comment */"); // correct
setLeadingComment("missing comment delimiters"); // wrong
setLeadingComment("/* unterminated traditional comment "); // wrong
setLeadingComment("/* broken\n traditional comment */"); // correct
setLeadingComment("// end-of-line comment\n"); // correct
setLeadingComment("// end-of-line comment without line terminator"); // correct
setLeadingComment("// broken\n end-of-line comment\n"); // wrong