Consistently version-bound functionality

[maleadt]

For functionality that's only available on certain LLVM versions, we currently either fail to look op the symbol (i.e. do nothing), error-out at run-time, or conditionally define wrappers at compilation time. That's a bit inconsistent, we should have a unified approach, e.g., tag functions with @required_llvm v"7.0" function... end. This could either be put on the wrappers, or be done by the wrapper generator script.