[WebAssembly] Improve libObject support for wasm imports and exports

Previously we had only supported the importing and
exporting of functions and globals.

Also, add usefull overload of getWasmSymbol() and
getNumberOfSymbols() in support of lld port.

Differential Revision: https://reviews.llvm.org/D33011

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302601 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed