[WebAssembly] Use dummy debug info in Emscripten SjLj

Summary:
D74269 added debug info to newly created instructions, including calls
to `malloc` and `free`, by taking debug info from existing instructions
around, whose debug info may or may not be empty.

But there are cases debug info is required by the IR verifier: when both
the caller and the callee functions have DISubprograms, meaning we
already have declarations to `malloc` or `free` with a DISubprogram
attached, newly created calls to `malloc` and `free` should have
non-empty debug info. This patch creates a non-empty dummy debug info in
this case to those calls to make the IR verifier pass.

Fixes https://bugs.llvm.org/show_bug.cgi?id=45461.

Reviewers: dschuff

Subscribers: aprantl, sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77784