[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.

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

llvm-svn: 343961