[clang] allow const structs/unions/arrays to be constant expressions for C

For code like:
struct foo { ... };
struct bar { struct foo foo; };
const struct foo my_foo = { ... };
struct bar my_bar = { .foo = my_foo };

Eli Friedman points out the relevant part of the C standard seems to
have some flexibility in what is considered a constant expression:

6.6 paragraph 10:
An implementation may accept other forms of constant expressions.

GCC 8 added support for these, so clang not supporting them has been a
constant thorn in the side of source code portability within the Linux
kernel.

Fixes: https://github.com/llvm/llvm-project/issues/44502

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D76096