Accept string literal decay in conditional operator

The cppcoreguidelines-pro-bounds-array-to-pointer-decay check currently
accepts:

const char *b = i ? "foo" : "foobar";

but not

const char *a = i ? "foo" : "bar";

This is because the AST is slightly different in the latter case (see
https://godbolt.org/z/MkHVvs).

This eliminates the inconsistency by making it accept the latter form
as well.

Fixes https://github.com/llvm/llvm-project/issues/31155.