GlobalTypeOptimization: Reorder fields in order to remove them (#6820)

Before, we only removed fields from the end of a struct. If we had, say

struct Foo {
  int x;
  int y;
  int z;
};

// Add no fields but inherit the parent's.
struct Bar : Foo {};

If y is only used in Bar, but never Foo, then we still kept it around, because
if we removed it from Foo we'd end up with Foo = {x, z}, Bar = {x, y, z} which
is invalid - Bar no longer extends Foo. But we can do this if we first reorder
the two:

struct Foo {
  int x;
  int z;
  int y; // now y is at the end
};

struct Bar : Foo {};

And the optimized form is

struct Foo {
  int x;
  int z;
};

struct Bar : Foo {
  int y; // now y is added in Bar
};

This lets us remove all fields possible in all cases AFAIK.

This situation is not super-common, as most fields are actually used both
up and down the hierarchy (if they are used at all), but testing on some
large real-world codebases, I see 10 fields removed in Java, 45 in Kotlin,
and 31 in Dart testcases.

The NFC change to src/wasm-type-ordering.h was needed for this to
compile.

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