Sema: Variable definitions cannot be __attribute__((alias))

Things that are OK:
  extern int var1 __attribute((alias("v1")));
  static int var2 __attribute((alias("v2")));

Things that are not OK:
  int var3 __attribute((alias("v3")));
  extern int var4 __attribute((alias("v4"))) = 4;

We choose to accpet:
  struct S { static int var5 __attribute((alias("v5"))); };

  This code causes assertion failues in GCC 4.8 and ICC 13.0.1, we have
  no reason to reject it.

This partially fixes PR22217.

llvm-svn: 226436