Support SFrame command-line and .cfi_section syntax (#149935)
This PR adds support for the llvm-mc command-line flag "--gsframe" and
adds ".sframe" to the legal values passed ".cfi_section". It plumbs the
option through the cfi handling code a fair amount. Code to support
actual section generation follows in a future PR.
These options match the gnu-assembler's support syntax for sframes, on
both the command line and in assembly files.
First in a series of changes that will allow llvm-mc to produce sframe
.cfi sections. For more information about sframes, see
https://sourceware.org/binutils/docs-2.44/sframe-spec.html
and the llvm-RFC here:
https://discourse.llvm.org/t/rfc-adding-sframe-support-to-llvm/86900