[XRay] Make the xray_instr_map section specification more correct

Summary:
We also add a test to show what currently happens when we create a
section per function and emit an xray_instr_map. This illustrates the
relationship (or lack thereof) between the per-function section and the
xray_instr_map section.

We also change the code generation slightly so that we don't always
create group sections, but rather only do so if a function where the
table is associated with is in a group.

Also in this change:

  - Remove the "merge" flag on the xray_instr_map section.
  - Test that we're generating the right table for comdat and non-comdat functions.

Reviewers: echristo, majnemer

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D23104

llvm-svn: 277580