[Asm] Don't list '@<type>' in diag when '@' is a comment

[olista01]

Summary

This fixes https://bugs.llvm.org//show_bug.cgi?id=31280

[SjoerdMeijer]

Looks good to me.

lib/MC/MCParser/ELFAsmParser.cpp

 }
 
 bool ELFAsmParser::maybeParseSectionType(StringRef &TypeName) {
   MCAsmLexer &L = getLexer();
   if (L.isNot(AsmToken::Comma))
     return false;
   Lex();
   if (L.isNot(AsmToken::At) && L.isNot(AsmToken::Percent) &&
-      L.isNot(AsmToken::String))
+      L.isNot(AsmToken::String)) {
+    if (L.getAllowAtInIdentifier())
       return TokError("expected '@<type>', '%<type>' or \"<type>\"");
+    else
+      return TokError("expected '%<type>' or \"<type>\"");
+  }
   if (!L.is(AsmToken::String))
     Lex();
   if (L.is(AsmToken::Integer)) {
     TypeName = getTok().getString();
     Lex();
   } else if (getParser().parseIdentifier(TypeName))
     return TokError("expected identifier in directive");
   return false;

test/MC/ELF/ARM/gnu-type-hash-diagnostics.s

 @ RUN: not llvm-mc -triple arm-elf -filetype asm -o /dev/null %s 2>&1 | FileCheck %s
 
   .syntax unified
 
   .type TYPE #32
 // CHECK: error: expected symbol type in directive
 // CHECK: .type TYPE #32
 // CHECK:             ^
 
+  // For ARM, the comment character is '@', so we don't list '@<type>' as a
+  // valid option.
+  .section "foo", "a", @progbits
+// CHECK: error: expected '%<type>' or "<type>"