[RemoveDIs] C API: Add before-dbg-record versions of IRBuilder position funcs (#92417)

Add `LLVMPositionBuilderBeforeDbgRecords` and
`LLVMPositionBuilderBeforeInstrAndDbgRecords` to `llvm/include/llvm-c/Core.h`
which behave the same as `LLVMPositionBuilder` and `LVMPositionBuilderBefore`
except that the position is set before debug records attached to the target
instruction (the existing functions set the insertion point to after any
attached debug records).

More info on debug records and the migration towards using them can be found
here: https://llvm.org/docs/RemoveDIsDebugInfo.html

The distinction is important in some situations. An important example is when
inserting a phi before another instruction which has debug records attached to
it (these come "before" the instruction). Inserting before the instruction but
after the debug records would result in having debug records before a phi, which
is illegal. That results in an assertion failure:

`llvm/lib/IR/Instruction.cpp:166: Assertion '!isa<PHINode>(this) && "Inserting
PHI after debug-records!"' failed.`

In llvm (C++) we've added bit to instruction iterators that carries around the
extra information. Adding dedicated functions seemed like the least invasive and
least suprising way to update the C API.

Update llvm/tools/llvm-c-test/debuginfo.c to test this functionality.

Update the OCaml bindings, the migration docs and release notes.

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