Compilation warnings introduced by https://github.com/JuliaLang/julia/pull/16848

[Scott Jones]

@Yuyichao's recent merge of #16848 has introduced a number of warnings (at least when compiling on Mac OS X).

    CC src/APInt-C.o

In file included from /j/julia/src/APInt-C.cpp:10:

./julia.h:78:9: warning: anonymous types declared in an anonymous union are an extension [-Wnested-anon-types]

        struct {

        ^

1 warning generated.

Also in the following places:

In file included from /j/julia/src/runtime_ccall.cpp:7:

In file included from /j/julia/src/codegen.cpp:122:

In file included from /j/julia/src/disasm.cpp:85:

In file included from /j/julia/src/debuginfo.cpp:48:

In file included from /j/julia/src/llvm-gcroot.cpp:22: