[WebAssembly] Use more specific type and simplify code.
[WebAssembly] Use more specific type and simplify code.

Instead of {Function,Global,Data}Symbol, use Defined{Function,Global,Data}
because undefined symbol should never reach this function.

Differential Revision: https://reviews.llvm.org/D43710

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@326275 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld