[WebAssembly] MC: Resolve aliases when creating provisional table entries

This change is useful for the upcoming addition of the symbol
table (D41954) since in that world aliases for given function
all share the same function index.

This change does not effect lld because it essentially ignores
the wasm "table".  The table exists only to the wasm objects
will validate and disassembly meaningfully.

Patch by Nicholas Wilson!

Differential Revision: https://reviews.llvm.org/D42095

llvm-svn: 323900