Implement [[eraseable]] attribute. When used with a function it instructs the AST...

Implement [[eraseable]] attribute. When used with a function it instructs the AST (which than transfers logic to the IR) that the function (or variable) is potentially unused (verified by checking the read count). This than propogates through the IR and prevents the IR from generating both the definition and global/function. The intrinsics system uses this as well to prevent intrinsic functions from being generated unless they're used.