[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

llvm-svn: 302492