flang regression needs LIBRARY_PATH

[scivision]

brew gist-logs <formula> link OR brew config AND brew doctor output

HOMEBREW_VERSION: 4.4.6
ORIGIN: https://github.com/Homebrew/brew
HEAD: cc0b5d6d05d1f3af5b2f53e9fbb1e7bfa0c332ab
Last commit: 30 hours ago
Core tap JSON: 18 Nov 16:32 UTC
Core cask tap JSON: 18 Nov 16:32 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_DISPLAY: /private/tmp/com.apple.launchd.ZtFLV6ifye/org.xquartz:0
HOMEBREW_EDITOR: nano
HOMEBREW_MAKE_JOBS: 8
Homebrew Ruby: 3.3.6 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.3.6/bin/ruby
CPU: octa-core 64-bit arm_blizzard_avalanche
Clang: 16.0.0 build 1600
Git: 2.47.0 => /opt/homebrew/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 15.1-arm64
CLT: 16.1.0.0.1.1729049160
Xcode: 16.1
Rosetta 2: false

Your system is ready to brew.

### Verification

- [X] My `brew doctor` output says `Your system is ready to brew.` and am still able to reproduce my issue.
- [X] I ran `brew update` and am still able to reproduce my issue.
- [X] I have resolved all warnings from `brew doctor` and that did not fix my problem.
- [X] I searched for recent similar issues at https://github.com/Homebrew/homebrew-core/issues?q=is%3Aissue and found no duplicates.

### What were you trying to do (and why)?

`flang` recently lost ability to compile any Fortran program unless LIBRARY_PATH is specified. Flang was previously working from Homebrew. The error is:

```sh
% flang-new main.f90

ld: library 'c++' not found
flang-new: error: linker command failed with exit code 1 (use -v to see invocation)

main.f90 is:

program main
end program

What happened (include all command output)?

The workaround (command that works) is:

% LIBRARY_PATH=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib flang-new main.f90  

What did you expect to happen?

I expected to be able to compile a trivial Fortran program, as Flang from Homebrew previously worked.

Step-by-step reproduction instructions (by running brew commands)

brew install flang

flang-new main.f90

[scivision]

related: #192611

[cho-m]

cc @carlocab

---

If flang was relying on LLVM config files (which add --sysroot to SDK), then the incorrect system directory could be related, which is probably caused by relative path from libexec/bin/flang-new:

❯ flang-new main.f90 -v 2>&1 | rg 'System configuration'
System configuration file directory: /opt/homebrew/Cellar/etc/clang