[WebAssembly] Respect `--no-mangle` in more locations

`--no-demangle` now also applies to the name section.  This change
was motivated by the rust team that have a slightly different name
mangling scheme to the standard C++ itanium one and prefer to do their
de-mangling as a post-link setp.

Patch by Alex Crichton!

Differential Revision: https://reviews.llvm.org/D54279

llvm-svn: 346516