[WebAssembly] MC: Use existing MCSymbol.Index field rather than inventing extra mapping
[WebAssembly] MC: Use existing MCSymbol.Index field rather than inventing extra mapping

MCSymbol has getIndex/setIndex which are implementation defined
and on other platforms are used to store the symbol table
index.  It makes sense to use this rather than invent a new
mapping.

Differential Revision: https://reviews.llvm.org/D46555

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@331705 91177308-0d34-0410-b5e6-96231b3b80d8

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