The callers of ParseStructDeclaration are not expecting it to

The callers of ParseStructDeclaration are not expecting it to
eat the terminating ;.  Fix one place where it did, allowing this
to compile without error:

struct x {
 int a;
 union {
  int b;
  float c;
 };

 int d;
};

This reduces diagnostics on PR1750 from 33 to 27.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@43437 91177308-0d34-0410-b5e6-96231b3b80d8