[clang][bytecode] Add support for (toplevel) array fillers.

[tbaederr]

This adds support for array fillers, for global, toplevel descriptors.

Toplevel meaning that this gets an array filler:

constexpr int foo[200] = {1};

But this does not:

struct S { int i[10] = {3}; };
constexpr S s{};

For multidimensional arrays, only the topmost dimension gets an array filler:

constexpr float arr_f[3][5] = {
    {1, 2, 3, 4, 5},
};

This means that this patch fixes e.g.
test/SemaCXX/large-array-init, but does not fix
test/CodeGenCXX/cxx11-initializer-aggregate.cpp, as that contains arrays such as:

  struct B { int n; int arr[1024 * 1024 * 1024 * 2u]; } b = {1, {2}};
  // ...
  unsigned char data_3[1024][1024][1024] = {{{0}}};
  // ...
  unsigned char data_12[1024][1024][1024] = {{{1}}};

additionally, adding array fillers regresses performance in the common case of no array fillers:
https://llvm-compile-time-tracker.com/compare.php?from=02052caa09b27b422c452a2e1be2e3bfed710156&to=5b654212c1442d814aa8ed1c8de960432e479cdd&stat=instructions:u