[Core] Do not reclaim absolute atoms in resolver.

This fixes a linker crash (found out while testing --gc-sections,
testcase provided by Rafael Avila de Espindola).
While this behaviour was found while testing ELF, it' not necessarily
ELF specific and this change is (apparently) harmless on all the
other drivers.

Differential Revision:  D7823
Reviewed by:    ruiu

llvm-svn: 230614