[Flang] Fix parsing error on loop count compiler directive

Fixes: https://github.com/llvm/llvm-project/issues/58731
Fixes: https://github.com/llvm/llvm-project/issues/56678

Only handles this form which was reported: `!DIR$ LOOP COUNT (n1[, n2]...)`

Differential Revision: https://reviews.llvm.org/D141976

Change-Id: I768c8cb9f289c5fc4adee1ef8262c6d827574f27