[flang] Fix some bugs that I noticed while running regression tests.

Add characters.h to consolidate all per-character functions
and reduce dependence on <cctype>.
Add omitted DIRECT= and UNFORMATTED= specifiers to INQUIRE statement.
Add Interval<>, rebase ProvenanceRange upon it, rebase CharPointerWithLength
and rename it ContiguousChars.
Reformat C++.

Original-commit: flang-compiler/f18@78c73c62badb3f2c32a931bbcdf0dec6b26b0312
Reviewed-on: https://github.com/flang-compiler/f18/pull/19
Tree-same-pre-rewrite: false