[Format] Don't derive pointers right based on space before method ref-qualifiers

The second space in `void foo() &` is always produced by clang-format,
and isn't evidence of any particular style.

Before this patch, it was considered evidence of PAS_Right, because
there is a space before a pointerlike ampersand.

This caused the following code to have "unstable" pointer alignment:
  void a() &;
  void b() &;
  int *x;
PAS_Left, Derive=false would produce 'int* x' with other lines unchanged.
But subsequent formatting with Derive=true would produce 'int *x' again.

Differential Revision: https://reviews.llvm.org/D118921