Move SymbolTable::addCombinedLTOObject() to LinkerDriver.

Also renames it LinkerDriver::compileBitcodeFiles.

The function doesn't logically belong to SymbolTable. We added this
function to the symbol table because symbol table used to be a
container of input files. This is no longer the case.

Differential Revision: https://reviews.llvm.org/D62291

llvm-svn: 361469