Fix "anonymous types declared in an anonymous union" warnings

[fingolfin]

They look like this:

    CC src/processor.o
In file included from /Users/mhorn/Projekte/Julia/julia.master/src/processor.cpp:10:
In file included from ./processor.h:5:
./julia.h:395:9: warning: anonymous types declared in an anonymous union are an extension [-Wnested-anon-types]
        struct {
        ^
./julia.h:405:9: warning: anonymous types declared in an anonymous union are an extension [-Wnested-anon-types]
        struct {
        ^
2 warnings generated.

and come from code that was introduced by @Keno in PR #43852.

But it turns out that the union is not used at all! So I'm simply removing
the offending union. Perhaps it is needed for some future work, but it should
be trivial to add it back if needed. If that happens, I suggest a comment
is added that explain why this looks similar to but has different layout
compared to the typedef _jl_purity_overrides_t also in julia.h.

[fingolfin]

Note that this also affects release-1.8, so ideally it should be backported.

[giordano]

Related issue: #44314

[fingolfin]

Can't this just be merged? There is zero risk, is there? At the worst, reverting it is trivial...

[vtjnash]

Might want to preserve a comment here? (similar to how we write ssaflags above in the file)

[staticfloat]

Keno says this is fine, as long as the structure is preserved in a comment, per Jameson's request.

[fingolfin]

I've now retained the union, commented out. I don't know how to make it similar to ssaflags, though.

[staticfloat]

Yeah, this looks fine to me. Thanks!