[WebAssembly] Base imports on Symtab. NFC.

Since imports are undefined symbols we know we can
find all of them my looking at the symbol table alone.
(i.e. imports cannot be have local binding).

This will be strictly faster and also allows us
to to remove a method from Symbol class

Differential Revision: https://reviews.llvm.org/D41304

llvm-svn: 320847