[flang] replace the stubs for translating terminal symbols to expressions;...

[flang] replace the stubs for translating terminal symbols to expressions; start pushing Value type into FIR; cleanup terminator succ_blocks; remove parser element from disassociate; run clang-format; etc.

Original-commit: flang-compiler/f18@c5561646ea4f72431174f28131a49426b8a03495
Reviewed-on: https://github.com/flang-compiler/f18/pull/334
Tree-same-pre-rewrite: false