[WebAssembly] MC: Make explicit our current lack of support for relocations...

[WebAssembly] MC: Make explicit our current lack of support for relocations against unnamed temporary symbols.

Add an explicit check before looking up symbol in SymbolIndices.
This was previously silently succeeding and returning zero for such
unnamed temporaries.

Differential Revision: https://reviews.llvm.org/D43365

llvm-svn: 325367