[PGO] Use linkonce_odr linkage for __profd_ variables in comdat groups

This fixes relocations against __profd_ symbols in discarded sections,
which is PR41380.

In general, instrumentation happens very early, and optimization and
inlining happens afterwards. The counters for a function are calculated
early, and after inlining, counters for an inlined function may be
widely referenced by other functions.

For C++ inline functions of all kinds (linkonce_odr &
available_externally mainly), instr profiling wants to deduplicate these
__profc_ and __profd_ globals. Otherwise the binary would be quite
large.

I made __profd_ and __profc_ comdat in r355044, but I chose to make
__profd_ internal. At the time, I was only dealing with coverage, and in
that case, none of the instrumentation needs to reference __profd_.
However, if you use PGO, then instrumentation passes add calls to
__llvm_profile_instrument_range which reference __profd_ globals. The
solution is to make these globals externally visible by using
linkonce_odr linkage for data as was done for counters.

This is safe because PGO adds a CFG hash to the names of the data and
counter globals, so if different TUs have different globals, they will
get different data and counter arrays.

Reviewers: xur, hans

Differential Revision: https://reviews.llvm.org/D67579

llvm-svn: 372020