LLVMFlang: Fix post-preprocess compile for fixed-form Fortran

LLVM Flang's preprocessor output is always valid fixed form Fortran with a fixed line length of 72. If an application tries to use fixed form with a different line length, this will fail in the post-preprocessing compilation step as they will pass a flag asking for a different fixed line length.

To fix this we can pass -ffixed-line-length-72 to the post-preprocess compilation stage. This will be ignored when the input is free form Fortran, and force to a 72 line length when the input is fixed.

Topic-rename: LLVMFlang-post-preprocessed