LTO: Link non-prevailing weak_odr or linkonce_odr globals into the combined...

LTO: Link non-prevailing weak_odr or linkonce_odr globals into the combined module with available_externally linkage.

These linkages mean that the ultimately prevailing symbol will have the same
semantics as any non-prevailing copy of the symbol, so we are free to ignore
the linker's resolution.

Differential Revision: https://reviews.llvm.org/D29367

llvm-svn: 293865