ELF: Create LTO.{cpp,h} and move LTO-related code to that file.

The code for LTO has been growing, so now is probably a good time to
move it to its own file. SymbolTable.cpp is for symbol table, and
because compiling bitcode files are semantically not a part of
symbol table, this is I think a good thing to do.

http://reviews.llvm.org/D18370

llvm-svn: 264091