Update for C and OpenCL C

[RunDevelopment]

Since some of the problems OpenCL C had were caused by C, I fixed them there. This also has the added benefit that some of the fixes in C also apply for languages derived from C (e.g. C++).

Changes

C:

• Struct names can now be preceded by an __attribute__ declaration.
• New keyword: __attribute__. This is a compiler-specific keyword and while I don't think Prism should include ever compiler-specific keyword, it seemed strange to support __attribute__((...)) between struct and the name of the structure but not the keyword itself.
• Allowed any number of spaces between a function name and (. This fix is also visible in C++.

OpenCL C:

• Updated constants. The list was incomplete and partly incorrect.
• Added support for half number literals.
• Added support for boolean literals.
• Removed the function-opencl-kernel token. I honestly don't know why this was added in the first place. All functions in C are highlighted by the preceding (, so there's no need for another huge list to maintain.