[llvm-c] Add C API methods to match size_t ConstantDataArray C++ API signatures (#84433)

Adds `LLVMConstStringInContext2` and `LLVMConstString2`, which are
identical to originals except that they use `size_t` for length. This is
a clone of
https://github.com/llvm/llvm-project/commit/35276f16e5a2cae0dfb49c0fbf874d4d2f177acc
and is needed for https://github.com/rust-lang/rust/pull/122000.

As an aside, the issue of 32 bit overflow on constants is present in the
C++ APIs as well. A few classes, e.g. `ConstantDataArray` and
`ConstantAggregateZero`, can hold 64-bit ArrayTypes but their length
accessors return 32-bit values. This means the same issue from the
original Rust report is also present in LLVM itself. Would it be a
reasonable goal to update all of these length methods & types to be
uint64_t, or would that be too breaking? Alternatively, we could use
safe fallible casts instead of implicit ones inside the accessors (if an
overflow does happen, the solution would be to use
`MyValue->getTy...