Revert "LTOCodeGenerator: turns linkonce(_odr) into weak_(odr) when present "MustPreserve" set"

This reverts commit r267644. Turning linkonce_odr into weak_odr is
a sementic change on Darwin: because of
`llvm::canBeOmittedFromSymbolTable()` we may emit the symbol as
weak_def_can_be_hidden instead of weak_definition.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 268606