[XRay] Add clang builtin for xray typed events.

Summary:
A clang builtin for xray typed events. Differs from
__xray_customevent(...) by the presence of a type tag that is vended by
compiler-rt in typical usage. This allows xray handlers to expand logged
events with their type description and plugins to process traced events
based on type.

This change depends on D45633 for the intrinsic definition.

Reviewers: dberris, pelikan, rnk, eizan

Subscribers: cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D45716

llvm-svn: 330220