[clang][bytecode] Implement bitcasts to composite types (#114776)

Only fixed-size, non-bitfield integral fields for now.

[clang][bytecode][NFC] Make Floating::bitcastToMemory const (#114777)

The other functions like this are also const.

[clang][bytecode] Implement bitcasts to floating-point values (#114485) (#114712)

This time I tested on big-endian hosts.

[clang][bytecode][NFC] Switch BitcastBuffer to SmallVector (#114677)

This is a little easier to work with since we are guaranteed that the
item type of the vector is byte sized and not something el

[clang][bytecode][NFC] Switch BitcastBuffer to SmallVector (#114677)

This is a little easier to work with since we are guaranteed that the
item type of the vector is byte sized and not something else.

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[clang][bytecode] Start implementing __builtin_bit_cast (#112126)

This is a subset of #68288, with hopefully narrower scope. It does not
support bitcasting to non-integral types yet.
Bitfields are

[clang][bytecode] Start implementing __builtin_bit_cast (#112126)

This is a subset of #68288, with hopefully narrower scope. It does not
support bitcasting to non-integral types yet.
Bitfields are supported, but only if they result in a full byte-sized
final buffer. It does not support casting from null-pointers yet or
casting from indeterminate bits.


The tests are from #68288 and partially from #74775.

The `BitcastBuffer` struct is currently always working in single bits,
but I plan to (try to) optimize this for the common full-byte case.

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