Reapply "[clang][CodeGen] Zero init unspecified fields in initializers in C" (#109898) (#110051)

This reverts commit d50eaac12f0cdfe27e942290942b06889ab12a8c. Also fixes
a bug calculating offsets f

Reapply "[clang][CodeGen] Zero init unspecified fields in initializers in C" (#109898) (#110051)

This reverts commit d50eaac12f0cdfe27e942290942b06889ab12a8c. Also fixes
a bug calculating offsets for bit fields in the original patch.

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[clang][CodeGen] Zero init unspecified fields in initializers in C (#97121)

When an initializer is provided to a variable, the Linux kernel relied
on the compiler to zero-initialize unspecified fie

[clang][CodeGen] Zero init unspecified fields in initializers in C (#97121)

When an initializer is provided to a variable, the Linux kernel relied
on the compiler to zero-initialize unspecified fields, as clarified in
https://www.spinics.net/lists/netdev/msg1007244.html.

But clang doesn't guarantee this:
1. For a union type, if an empty initializer is given, clang only
initializes bytes for the first field, left bytes for other (larger)
fields are marked as undef. Accessing those undef bytes can lead
to undefined behaviors.
2. For a union type, if an initializer explicitly sets a field, left
bytes for other (larger) fields are marked as undef.
3. When an initializer is given, clang doesn't zero initialize padding.

So this patch makes the following change:
1. In C, when an initializer is provided for a variable, zero-initialize
undef and padding fields in the initializer.
2. Document the change in LanguageExtensions.rst.

As suggested in
https://github.com/llvm/llvm-project/issues/78034#issuecomment-2183437928,
the change isn't required by C23, but it's standards conforming to do
so.

Fixes: https://github.com/llvm/llvm-project/issues/97459

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