[XRay][compiler-rt] Add function id utilities for XRay

Summary:
This change allows us to provide users and implementers of XRay handlers
a means of converting XRay function id's to addresses. This, in
combination with the facilities provided in D32695, allows users to find
out:

  - How many function id's there are defined in the current binary.
  - Get the address of the function associated with this function id.
  - Patch only specific functions according to their requirements.

While we don't directly provide symbolization support in XRay, having
the function's address lets users determine this information easily
either during runtime, or offline with tools like 'addr2line'.

Reviewers: dblaikie, echristo, pelikan

Subscribers: kpw, llvm-commits

Differential Revision: https://reviews.llvm.org/D32846

llvm-svn: 302210