[XRay] Add __xray_customeevent(...) as a clang-supported builtin

Description

[XRay] Add __xray_customeevent(...) as a clang-supported builtin

Summary:
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).

Reviewers: echristo, rsmith

Subscribers: mehdi_amini, pelikan, lrl, cfe-commits

Differential Revision: https://reviews.llvm.org/D30018