Add LFortran CI workflow

[joewallwork]

Closes #600
Merges into #595

Let's see how far it gets.

[joewallwork]

@copilot Fix the error with building LFortran in the CI

[Copilot]

@copilot Fix the error with building LFortran in the CI

Fixed in the latest commit. The workflow was installing libunwind8 (runtime-only) but LFortran's CMake build requires libunwind-dev which provides the header files (LIBUNWIND_INCLUDE_DIR) and development libraries (LIBUNWIND_LIBRARY) needed to compile with stacktrace support.

[joewallwork]

@copilot Fix error building FTorch with PFUnit in Flang CI workflow

[Copilot]

@copilot Fix error building FTorch with PFUnit in Flang CI workflow

Fixed in f43572e.

[joewallwork]

@certik I'm trying to build FTorch with LFortran (with LLVM Clang and Clang++ as the C and C++ compilers) and made at least some progress in this PR. The error messages that I get from LFortran during the FTorch build are difficult for me to interpret. Do you have any intuition as to what's going on here?
https://github.com/Cambridge-ICCS/FTorch/actions/runs/27267848511/job/80529646001?pr=601