Allow .alt_entry symbols to pass the .cfi nesting check (llvm#82268)

A symbol with an `N_ALT_ENTRY` attribute may be defined in the middle of
a subsection, so it is reasonable to opt them out of the
`.cfi_{start,end}proc` nesting check.

Fixes: llvm#82261

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -44,6 +44,7 @@
 #include "llvm/MC/MCSection.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSymbol.h"
+#include "llvm/MC/MCSymbolMachO.h"
 #include "llvm/MC/MCTargetOptions.h"
 #include "llvm/MC/MCValue.h"
 #include "llvm/Support/Casting.h"
@@ -1950,7 +1951,8 @@ bool AsmParser::parseStatement(ParseStatementInfo &Info,
       Lex();
     }
 
-    if (MAI.hasSubsectionsViaSymbols() && CFIStartProcLoc && Sym->isExternal())
+    if (MAI.hasSubsectionsViaSymbols() && CFIStartProcLoc &&
+        Sym->isExternal() && !cast<MCSymbolMachO>(Sym)->isAltEntry())
       return Error(StartTokLoc, "non-private labels cannot appear between "
                                 ".cfi_startproc / .cfi_endproc pairs") &&
              Error(*CFIStartProcLoc, "previous .cfi_startproc was here");

llvm/test/MC/AArch64/cfi-bad-nesting-darwin.s

@@ -8,6 +8,10 @@
 	.p2align	2
 _locomotive:
 	.cfi_startproc
+	; An N_ALT_ENTRY symbol can be defined in the middle of a subsection, so
+	; these are opted out of the .cfi_{start,end}proc nesting check.
+	.alt_entry _engineer
+_engineer:
 	ret
 
 	; It is invalid to have a non-private label between .cfi_startproc and
@@ -17,7 +21,7 @@ _locomotive:
 	.p2align	2
 _caboose:
 ; DARWIN: [[#@LINE-1]]:1: error: non-private labels cannot appear between .cfi_startproc / .cfi_endproc pairs
-; DARWIN: [[#@LINE-10]]:2: error: previous .cfi_startproc was here
+; DARWIN: [[#@LINE-14]]:2: error: previous .cfi_startproc was here
 	ret
 	.cfi_endproc