crucible-llvm: Don't crash when simulating llvm.dbg.assign intrinsic

[RyanGlScott]

The llvm.dbg.assign intrinsic was introduced in LLVM 16 and replaces the llvm.dbg.declare intrinsic in LLVM 17. In order to support LLVM 17, we will need to teach crucible-llvm about it. For example, crux-llvm will fail to process this program (when building on top of the llvm-pretty-bc-parser changes from GaloisInc/llvm-pretty-bc-parser#261):

__attribute__((noinline)) int foo(int x[2]) {
  return x[0];
}

int main(void) {
  int x[2] = { 0, 0 };
  return foo(x);
}

$ PATH=~/Software/clang+llvm-17.0.2/bin:$PATH cabal run exe:crux-llvm -- test.c
[Crux] Using pointer width: 64 for file crux-build/crux~test.bc
[Crux] Simulating function main
[Crux] Attempting to prove verification conditions.
[Crux] *** debug executable: results/test/debug-0
[Crux] *** break on line: 0
[Crux] Found counterexample for verification goal
[Crux]   test.c:0:0: error: in main
[Crux]   unsupported LLVM value: ValMd (ValMdValue (Typed {typedType = PrimType (Integer 1), typedValue = ValUndef})) of type metadata
[Crux] Goal status:
[Crux]   Total: 1
[Crux]   Proved: 0
[Crux]   Disproved: 1
[Crux]   Incomplete: 0
[Crux]   Unknown: 0
[Crux] Overall status: Invalid.

In order to make this work, we need to alter the special-casing for llvm.dbg.* intrinsics here. For now, it will suffice to skip calls to llvm.dbg.assign intrinsic, just like most other llvm.dbg.* intrinsics are skipped. If someone wants to reason about the contents of llvm.dbg.assign in a way similar to how other code reasons about llvm.dbg.declare (e.g., how it's done in SAW here), then someone will need to add an LLVM_Dbg_Assign data constructor (similar to the existing LLVM_Dbg_Declare data constructor). I don't have a need for LLVM_Dbg_Assign just yet, so I propose not doing this until someone needs it.