Implement trampoline and handler for typed xray event tracing.

Summary:
Compiler-rt support first before defining the __xray_typedevent() lowering in
llvm. I'm looking for some early feedback before I touch much more code.

Reviewers: dberris

Subscribers: delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D43668

llvm-svn: 330218