[CoroSplit][DebugInfo] Don't use entry_value for async args in 32-bit targets

Only X86_64 and ARM64 have a reserved register for async arguments, and so the
debugger is only able to handle those targets. For other architectures, we use a
non-entry-value expression and let the debugger do its best with that.

Differential Revision: https://reviews.llvm.org/D158638