We define the __xray_customeevent builtin that gets translated to
IR calls to the correct intrinsic. The default implementation of this is
a no-op function. The codegen side of this follows the following logic:
- When -fxray-instrument is not provided in the driver, we elide all calls to __xray_customevent.
- When -fxray-instrument is enabled and a function is marked as "never instrumented", we elide all calls to __xray_customevent in that function; if either marked as "always instrumented" or subject to threshold-based instrumentation, we emit a call to the llvm.xray.customevent intrinsic from LLVM for each __xray_customevent occurrence in the function.
This change depends on D27503 (to land in LLVM first).