[XRay][compiler-rt] Return the pointer associated with the function instead of the sled

Summary:
XRay has erroneously been returning the address of the first sled in the
instrumentation map for a function id instead of the (runtime-relocated)
functison address. This causes confusion and issues for applications
where:

  - The first sled in the function may not be an entry sled (due to
    re-ordering or some other reason).
  - The caller attempts to find a symbol associated with the pointer at
    runtime, because the sled may not be exactly where the function's
    known address is (in case of inlined functions or those that have an
    external definition for symbols).

This fixes http://llvm.org/PR34340.

Reviewers: eizan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37202

llvm-svn: 311871