The Linux kernel makes use of %c<digit> "output template" in extended
inline assembly, particularly with asm goto for static key runtime
patching. The inline assembly uses the address of a global variable as
input to the extended inline assembly.
There's generic handling of %c in AsmPrinter::PrintAsmOperand() for
immediates. I considered extending support for global addresses there,
but it seems that X86AsmPrinter::PrintAsmOperand() has quite a bit of
custom logic for printing global addresses (see printSymbolOperand() in