[WebAssembly] Simplify generation of "names" section

Simplify generation of "names" section by simply iterating
over the DefinedFunctions array.

This even fixes some bugs, judging by the test changes required.
Some tests are asserting that functions are named multiple times,
other tests are asserting that the "names" section contains the
function's alias rather than its original name

Patch by Nicholas Wilson!

Differential Revision: https://reviews.llvm.org/D42076

llvm-svn: 322751