[WebAssembly] Use function/global index space in WasmSymbol

It is useful for the symbol to contain the index of the
function of global it represents in the function/global
index space.

For imports we also store the import index so that the
linker can find, for example, the signature of the
corresponding function, which is defined by the import

In the long run we need to decide whether this API
surface should be closer to binary (where imported
functions are seperate) or the wasm spec (where the
function index space is unified).

Differential Revision: https://reviews.llvm.org/D38189

llvm-svn: 314230