[x86/asm] Let EmitMSInlineAsmStr() handle variants too

This is preparation for D113707, where I want to make `-masm=intel`
emit `asm inteldialect` instructions.

`{movq %rbx, %rax|mov rax, rbx}` is supposed to evaluate to the bit
between { and | for att and to the bit between | and } for intel.
Since intel will become `asm inteldialect`, which alls EmitMSInlineAsmStr(),
EmitMSInlineAsmStr() has to support variants as well.

(clang translates `{...|...}` to `$(...$|...$)`. I'm not sure why
it doesn't just send along only the first `...` or the second `...`
to LLVM, but given the notes in PR23933 let's not do a big
reorganization in this codepath.)

Differential Revision: https://reviews.llvm.org/D113932