QIR generation for math functions should use LLVM intrinsics where possible, use `rt` functions otherwise

[swernli]

Right now the math functions in Q# are treated as if they are quantum intrinsics, and become __quantum__qis__* declarations. Instead, the compiler should insert calls to the appropriate LLVM intrinsic trig functions or combinations thereof, and for those that can't it should treat them as functions supported by the standard library and named with the __quantum__rt__* pattern.