[asm] Merge EmitMSInlineAsmStr() and EmitGCCInlineAsmStr()

This basically reverts 1778831a, which split them.
Since they were split 9 years ago, EmitGCCInlineAsmStr() grew a bunch of
features that usually weren't added to EmitMSInlineAsmStr(), and
that was usually a mistake.  D71677, D113932, D114167 are all examples
of where things were backported to EmitMSInlineAsmStr().

The names were also not great. EmitMSInlineAsmStr() used to be called for `asm
inteldialect`, which clang produces for Microsoft-style __asm { ... } blocks as
well for GCC-style __asm__ / asm statements with -masm=intel. On the other hand,
EmitGCCInlineAsmStr() used to be called for `asm`, whic clang produces for
GCC-style __asm__ / asm statements with -masm=att (the default).

It's also less code (23 insertions, 188 deletions).

No behavior change.

Differential Revision: https://reviews.llvm.org/D114330