[XRAY] [compiler-rt] [NFC] Fixing the bit twiddling of Function Id in FDR logging mode.

Summary:
Fixing a bug I found when testing a reader for the FDR format. Function ID is
now correctly packed into the 28 bits which are documented for it instead of being
masked to all ones.

Reviewers: dberris, pelikan, eugenis

Reviewed By: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29698

llvm-svn: 294563