Object, LTO: Add target triple to irsymtab and LTO API.

Object, LTO: Add target triple to irsymtab and LTO API.

Start using it in LLD to avoid needing to read bitcode again just to get the
target triple, and in llvm-lto2 to avoid printing symbol table information
that is inappropriate for the target.

Differential Revision: https://reviews.llvm.org/D32038

llvm-svn: 300300