Interpret "flang" and "flang-new" as LLVM flang

[rouson]

Description

Issue #1086 calls for a better method of compiler identification than checking the compiler name.
The current issue proposes an intermediate solution that presumably is straightforward to implement.

Possible Solution

I suggest that

1. flang and flang-new both resolve to LLVM Flang and
2. A new name, e.g., flang-classic, resolve to Classic Flang.

This breaks existing workflows in a way that hopefully has minimal cost to fix.
Users could, for example, create an executable flang-classic wrapper script with the following contents:

#!/bin/bash
<insert-path-to-classic-flang> "$@"

Additional Information

Additional considerations:

1. The LLVM Flang README.md file states, "... the plan is to replace Classic Flang with the new Flang in the future."
2. The Classic Flang Wiki describes Classic Flang as a "Fortran 2003/2008 Compiler," whereas
3. LLVM Flang 19 supports most of Fortran 2018 (with the primary exception being parallel features that Classic Flang also doesn't support),

where item 1 suggests that the change proposed in this issue is inevitable and items 2 and 3 suggest that now is a reasonable time to make the change.

Feedback is welcome.