[XRay][lib] Support and temporarily skip over CustomEvent records

Summary:
In D30630 we will start writing custom event records. To avoid breaking
the tools that read the FDR mode records, we skip over these records.
To support these custom event records more effectively, we will have to
expose them in the trace loading API. Those changes will be forthcoming.

Reviewers: kpw, pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33032

llvm-svn: 302856