Pack symbols a bit more.

Before this patch:

Symbol 56
Defined 80
Undefined 56
SharedSymbol 88
LazyArchive 72
LazyObject 56

With this patch

Symbol 48
Defined 72
Undefined 48
SharedSymbol 80
LazyArchive 64
LazyObject 48

The result is that peak allocation when linking chromium (according to
heaptrack) goes from 578 to 568 MB.

llvm-svn: 330874