[pseudo] Add crude heuristics to choose taken preprocessor branches.

In files where different preprocessing paths are possible, our goal is to
choose a preprocessed token sequence which we can parse that pins down as much
of the grammatical structure as possible.
This forms the "primary parse", and the not-taken branches get parsed later,
and are constrained to be compatible with the primary parse.

Concretely:
  int x =
    #ifdef // TAKEN
      2 + 2 + 2 // determined during primary parse to be an expression
    #else
      2 // constrained to be an expression during a secondary parse
    #endif
    ;

Differential Revision: https://reviews.llvm.org/D121165