This patch introduces an LLVM intrinsic and a target opcode for custom event logging in XRay. Initially, its use case will be to allow users of XRay to log some type of string ("poor man's printf"). The target opcode compiles to a noop sled large enough to enable calling through to a runtime-determined relative function call. At runtime, when X-Ray is enabled, the sled is replaced by compiler-rt with a trampoline to the logic for creating the custom log entries.
Future patches will implement the compiler-rt parts and clang-side support for emitting the IR corresponding to this intrinsic.