[lld][WebAssembly] Take advantage of extended const expressions when available

In particular we use these in two places:

1. When building PIC code we no longer need to combine output segments
   into a single segment that can be initialized at `__memory_base`.
   Instead each segment can encode its offset from `__memory_base` in
   its initializer.  e.g.

```
(i32.add (global.get __memory_base) (i32.const offset)
```

2. When building PIC code we no longer need to relocation internalized
   global addresses.  We can just initialize them with their correct
   offsets.

Differential Revision: https://reviews.llvm.org/D121420