make asm diagnostic instruction optional
`DiagnosticInfoInlineAsm::getInstruction` may return a null pointer, so
the instruction shouldn't be blindly unwrapped.
Reopening from #55193. I was unable to trigger the assertion on Windows after rebasing.
Fixes#23458.
Fixes#55216.
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc guide.