[C API] Fix LLVMGetOrdering/LLVMIsAtomicSingleThread for fence/memory instrs (#65228)

Fixes https://github.com/llvm/llvm-project/issues/65227

LLVMGetOrdering previously did not support Fence instructions, and
calling it on a fence would lead to a bad cast as it
assumed a load/store, or an AtomicRMWInst. This would either read a
garbage memory order, or assertion

LLVMIsAtomicSingleThread did not support either Fence instructions,
loads, or stores, and would similarly lead to a bad cast.
It happened to work out since the relevant types all have their synch
scope ID at the same offset, but it still should be fixed

These cases are now fixed for the C API, and tests for these
instructions are added. The echo test utility now also supports cloning
Fence instructions, which it did not previously


From what I can tell, there's no unified API to pull
`getOrdering`/`getSyncScopeID` from, and instead requires casting to
individual types: if there is a better way of handling this I can switch
to that

GitOrigin-RevId: d222c5ec47a09a3e120000e4a1006f4da7741256
3 files changed