[MC] maybeParseSectionType: test CommentString instead of AllowAtInIdentifier

Rework https://reviews.llvm.org/D31026
AllowAtInIdentifier is a misnomer: it should be false for ELF targets,
but is currently true as a hack to parse expr@specifier.

Author: MaskRay

llvm/lib/MC/MCParser/ELFAsmParser.cpp

@@ -401,10 +401,10 @@ bool ELFAsmParser::maybeParseSectionType(StringRef &TypeName) {
   Lex();
   if (L.isNot(AsmToken::At) && L.isNot(AsmToken::Percent) &&
       L.isNot(AsmToken::String)) {
-    if (L.getAllowAtInIdentifier())
-      return TokError("expected '@<type>', '%<type>' or \"<type>\"");
-    else
+    if (getContext().getAsmInfo()->getCommentString().starts_with('@'))
       return TokError("expected '%<type>' or \"<type>\"");
+    else
+      return TokError("expected '@<type>', '%<type>' or \"<type>\"");
   }
   if (!L.is(AsmToken::String))
     Lex();

llvm/test/MC/ELF/gnu-type-diagnostics.s

@@ -15,4 +15,5 @@
 // CHECK: .type symbol 32
 // CHECK:              ^
 
-
+.section "foo", "a", !progbits
+// CHECK: [[#@LINE-1]]:22: error: expected '@<type>', '%<type>' or "<type>"