[WebAssembly] Write minimal types section

Don't include type signatures that are not referenced by
some relocation.

We don't include this in the -gc-sections settings since
we are always building the type section from scratch,
just like we do the table elements.

In the future we might want to unify the relocation
processing which is currently done once for gc-sections
and then again for building the sympathetic type and
table sections.

Differential Revision: https://reviews.llvm.org/D42747

llvm-svn: 323931