[WebAssembly] Return ArrayRef's rather than const std::vector&
[WebAssembly] Return ArrayRef's rather than const std::vector&

From working on lld I've learned this is generally the
preferred way for several reasons (e.g. more concise, improves
encapsulation).

Differential Revision: https://reviews.llvm.org/D41265

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@320906 91177308-0d34-0410-b5e6-96231b3b80d8

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