[AsmParser] Return an error in the case of empty symbol ref in an expression

The following instruction:

> str q28, [x0, #1*6*4*@]

contains a @ which is parsed as an empty symbol. The parser returns true
but has no error, so the assembler continues by ignoring the
instruction.

rdar://44534987

Differential Revision: https://reviews.llvm.org/D52645

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343961 91177308-0d34-0410-b5e6-96231b3b80d8

Author: francisvm

lib/MC/MCParser/AsmParser.cpp

@@ -1100,7 +1100,7 @@ bool AsmParser::parsePrimaryExpr(const MCExpr *&Res, SMLoc &EndLoc) {
     // This is a symbol reference.
     StringRef SymbolName = Identifier;
     if (SymbolName.empty())
-      return true;
+      return Error(getLexer().getLoc(), "expected a symbol reference");
 
     MCSymbolRefExpr::VariantKind Variant = MCSymbolRefExpr::VK_None;
 

test/MC/AArch64/expr-bad-symbol.s

@@ -0,0 +1,6 @@
+// RUN: not llvm-mc -triple aarch64-- %s 2>&1 | FileCheck %s
+
+  .text
+_foo:
+  str q28, [x0, #1*6*4*@]
+// CHECK: error: expected a symbol reference