In the LLVM-C library, there is currently no way to get information about a
DbgRecord - which is the new way to attach debug information to llvm
instructions.

We can only iterate on debug records with LLVMGetFirstDbgRecord/
LLVMGetLastDbgRecord/LLVMGetNextDbgRecord, but there is no way to read
information.

This PR adds utility functions to read DbgRecord information.

Allow LLVMGetVolatile() to work with any kind of Instruction, rather
than only memory instructions that accept a volatile flag. For
instructions that can never be volatile, the function now return false
instead of asserting. This matches the behavior of
`Instruction::isVolatile()` in the C++ API.

Add `LLVMGetOrInsertFunction` to the C API as a thin wrapper over
`Module::getOrInsertFunction`, upstreamed from
[rustc](https://github.com/rust-lang/rust/blob/d773bd07d63a74adcf25ea5f4aae986be94cac5e/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp#L203).
It provides a single-call way to get or create a function declaration,
avoiding `LLVMGetNamedFunction` + `LLVMAddFunction` and is more
idiomatic.

I'm using the LLVM C bindings through the llvm-sys rust crate, and
noticed that LLVMGetFirstDbgRecord and LLVMGetLastDbgRecord are
segfault-ing when called on instructions without debug markers. I found
out it's missing a null pointer check. This PR fixes the issue.

This change allows globals to have multiple metadata attached. The
GlobalSetMetadata function only allows only one and is clobbered if
more metadata is attempted to be added. The addDebugInfo
function calls addMetadata. This is needed because some languages have
global structs containing lots of compiler-generated globals.

This was added to the C++ API in
https://github.com/llvm/llvm-project/pull/111419 so this change adds
accessors in the C API, along with a couple tests.

This flag was used to let us incrementally introduce debug records
into LLVM, however everything is now using records. It serves no
purpose now, so delete it.

Currently, GlobalObject has an "alignment" property... but it's
basically nonsense: alignment doesn't mean the same thing for variables
and functions, and it's completely meaningless for ifuncs.

This "removes" (actually marking protected) the methods from
GlobalObject, adds the relevant methods to Function and GlobalVariable,
and adjusts the code appropriately.

This should make future alignment-related cleanups easier.

This is for consistency with #139401.

With this change, some callers get to use StringRef::starts_with.

I'm planning to teach getAsmString to return StringRef also, but
I'ld like to keep that separate from this patch.