[InstrProfiling] Make __profd_ unconditionally private for ELF

For ELF, since all counters/data are in a section group (either `comdat any` or
`comdat noduplicates`), and the signature for `comdat any` is `__profc_`, the
D1003372 optimization prerequisite (linker GC cannot discard data variables
while the text section is retained) is always satisified, we can make __profd_
unconditionally private.

Reviewed By: davidxl, rnk

Differential Revision: https://reviews.llvm.org/D103717