[WebAssembly] Optimise relocation processing. NFC.

This is a rebased version https://reviews.llvm.org/D42176 which is patch
by Nicolas Wilson.

Addresses issue:
https://github.com/WebAssembly/tool-conventions/issues/32, and
https://bugs.llvm.org/show_bug.cgi?id=38650

Previously, for each function/segment we iterated over every relocation
to find the relevant ones, which is an n^2 operation. Now, we just make
a single pass.

Differential Revision: https://reviews.llvm.org/D51063

llvm-svn: 340428