[PGO] Move __profc_ and __profvp_ from their own comdat groups to __profd_'s comdat group

D68041 placed `__profc_`,  `__profd_` and (if exists) `__profvp_` in different comdat groups.
There are some issues:

* Cost: one or two additional section headers (`.group` section(s)): 64 or 128 bytes on ELF64.
* `__profc_`,  `__profd_` and (if exists) `__profvp_` should be retained or
  discarded. Placing them into separate comdat groups is conceptually inferior.
* If the prevailing group does not include `__profvp_` (value profiling not
  used) but a non-prevailing group from another translation unit has `__profvp_`
  (the function is inlined into another and triggers value profiling), there
  will be a stray `__profvp_` if --gc-sections is not enabled.
  This has been fixed by 3d6f5301.

Actually, we can reuse an existing symbol (we choose `__profd_`) as the group
signature to avoid a string in the string table (the sole reason that D68041
could improve code size is that `__profv_` was an otherwise unused symbol which
wasted string table space). This saves one or two section headers.

For a -DCMAKE_BUILD_TYPE=Release -DLLVM_BUILD_INSTRUMENTED=IR build, `ninja
clang lld`, the patch has saved 10.5MiB (2.2%) for the total .o size.

Reviewed By: davidxl

Differential Revision: https://reviews.llvm.org/D84723