[ms] [llvm-ml] Add support for anonymous labels (`@@`, `@B`, `@F`)

ml.exe and ml64.exe support the use of anonymous labels, with @B and @F referring to the previous and next anonymous label respectively.

We add similar support to llvm-ml, with the exception that we are unable to emit an error message for an @F expression not followed by a @@ label.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D128944