Revert "[MC][AsmParser] Diagnose improperly nested .cfi frames"

This reverts commit 4323da9.

This broke building libffi for ARM on Windows (and probably Darwin),
where one extern function intentionally falls through to another
one, while sharing one CFI region.

As long as one isn't using .subsections_via_symbols on MachO,
this probably shouldn't be a hard error.

Secondly, the tested pattern only produces an error on MachO and
COFF targets, but not for ELF, making the error case even more
inconsistent.

Reverting this commit for now, to figure out the best way forward.

lld/test/COFF/gc-dwarf-eh.s

@@ -13,9 +13,9 @@
 	.def	_main; .scl	2; .type	32; .endef
 	.section	.text,"xr",one_only,_main
 	.globl	_main
-_main:
 	.cfi_startproc
 	.cfi_personality 0, ___gxx_personality_v0
+_main:
 	xorl	%eax, %eax
 	ret
 	.cfi_endproc
@@ -29,8 +29,8 @@ ___gxx_personality_v0:
 	.def	_unused; .scl	2; .type	32; .endef
 	.section	.text,"xr",one_only,_unused
 	.globl	_unused
-_unused:
 	.cfi_startproc
 	.cfi_personality 0, ___gxx_personality_v0
+_unused:
 	ret
 	.cfi_endproc

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -125,7 +125,6 @@ class AsmParser : public MCAsmParser {
   void *SavedDiagContext;
   std::unique_ptr<MCAsmParserExtension> PlatformParser;
   SMLoc StartTokLoc;
-  std::optional<SMLoc> CFIStartProcLoc;
 
   /// This is the current buffer index we're lexing from as managed by the
   /// SourceMgr object.
@@ -1950,11 +1949,6 @@ bool AsmParser::parseStatement(ParseStatementInfo &Info,
       Lex();
     }
 
-    if (CFIStartProcLoc && Sym->isExternal())
-      return Error(StartTokLoc, "non-private labels cannot appear between "
-                                ".cfi_startproc / .cfi_endproc pairs") &&
-             Error(*CFIStartProcLoc, "previous .cfi_startproc was here");
-
     if (discardLTOSymbol(IDVal))
       return false;
 
@@ -4199,8 +4193,6 @@ bool AsmParser::parseDirectiveCFISections() {
 /// parseDirectiveCFIStartProc
 /// ::= .cfi_startproc [simple]
 bool AsmParser::parseDirectiveCFIStartProc() {
-  CFIStartProcLoc = StartTokLoc;
-
   StringRef Simple;
   if (!parseOptionalToken(AsmToken::EndOfStatement)) {
     if (check(parseIdentifier(Simple) || Simple != "simple",
@@ -4221,11 +4213,8 @@ bool AsmParser::parseDirectiveCFIStartProc() {
 /// parseDirectiveCFIEndProc
 /// ::= .cfi_endproc
 bool AsmParser::parseDirectiveCFIEndProc() {
-  CFIStartProcLoc = std::nullopt;
-
   if (parseEOL())
     return true;
-
   getStreamer().emitCFIEndProc();
   return false;
 }