[Clang interpreter] Avoid storing pointers at unaligned locations

The Clang interpreter's bytecode uses a packed stream of bytes
representation, but also wants to have some opcodes take pointers as
arguments, which are currently embedded in the bytecode directly.

However, CHERI, and thus Arm's upcoming experimental Morello prototype,
provide spatial memory safety for C/C++ by implementing language-level
(and sub-language-level) pointers as capabilities, which track bounds,
permissions and validity in hardware. This uses tagged memory with a
single tag bit at every capability-aligned address, and so storing
pointers to unaligned addresses results in the tag being stripped,
leading to a tag fault when the pointer is ultimately dereferenced at a
later point.

In order to support a stricter C/C++ implementation like CHERI, we no
longer store pointers directly in the bytecode, instead storing them in
a table and embedding the index in the bytecode.

Reviewed By: nand

Differential Revision: https://reviews.llvm.org/D97606