[flang] Replace formatting of CharBlock & string

Preserve generated strings until consumed by formatting.

bugfix from premature push

Address review comments

Last fix(?)

Use %s formatting for CharBlocks and strings

Use new formatting and fix usage of std::forward<>()

Use forward_list rather than vector to save strings

Original-commit: flang-compiler/f18@8ea478420fd21a26a77c6b4e05c73529a566f4a5
Reviewed-on: https://github.com/flang-compiler/f18/pull/444
Tree-same-pre-rewrite: false