[flang] Module file improvements

Verify that checksums are correct when reading a module file.

Don't write a module file if its current contents are correct.

Don't make .mod files read-only. It doesn't do much to prevent
users from editing them, checksum verification will detect when
it happens, and having them read-only causes problems if you then
compile with another compiler (e.g. PGI).

Original-commit: flang-compiler/f18@21d82aef6e8836f4cc0736d4eb2b59abdfa6fbc0
Reviewed-on: https://github.com/flang-compiler/f18/pull/164
Tree-same-pre-rewrite: false