Fix #479: C backend: preserve case in union member names

[andreasabel]

Fix #479: C: case-sensitive labels: preserve case in union member names.

Java does not suffer case-sensitive labels as each label becomes a class in its own file.
Thus, we check this and throw an error.
Consequently, test 479_LabelsCaseSensitive is broken for the java backends.

Breaking change: this might break C client code.

Fixes #479

• labels with same letters but different cases break the compilation on the c backend #479

[andreasabel]

@ivanperez-keera : Would this change in the C backend affect you? What is your opinion?
In short, union member for EInt used to be eint_, now it is eInt_. So the first letter is made lowercase, but the rest no longer, instead its case is preserved.

[andreasabel]

This adds java/479_LabelsCaseSensitive to the broken tests:

• Pygments backend:Pygments Backend
• Parameterized tests:Java (with jflex and line numbers):Java (with jflex and line numbers):479_LabelsCaseSensitive
• Parameterized tests:Java (with jflex):Java (with jflex):479_LabelsCaseSensitive
• Parameterized tests:Java (with namespace):Java (with namespace):479_LabelsCaseSensitive
• Parameterized tests:Java (with line numbers):Java (with line numbers):479_LabelsCaseSensitive
• Parameterized tests:Haskell/GADT:Haskell/GADT:289_LexerKeywords
• Parameterized tests:Java (with antlr):Java (with antlr):479_LabelsCaseSensitive
• Parameterized tests:OCaml:OCaml:289_LexerKeywords
• Parameterized tests:OCaml:OCaml:202_comments
• Parameterized tests:OCaml/Menhir:OCaml/Menhir:289_LexerKeywords
• Parameterized tests:OCaml/Menhir:OCaml/Menhir:202_comments
• Parameterized tests:Java:Java:479_LabelsCaseSensitive