[llvm][IR] Do not place constants with static relocations in a mergeable section

This patch provides two major changes:

1. Add getRelocationInfo to check if a constant will have static, dynamic, or
   no relocations. (Also rename the original needsRelocation to needsDynamicRelocation.)
2. Only allow a constant with no relocations (static or dynamic) to be placed
   in a mergeable section.

This will allow unused symbols that contain static relocations and happen to
fit in mergeable constant sections (.rodata.cstN) to instead be placed in
unique-named sections if -fdata-sections is used and subsequently garbage collected
by --gc-sections.

See https://lists.llvm.org/pipermail/llvm-dev/2021-February/148281.html.

Differential Revision: https://reviews.llvm.org/D95960