[flang] Make lto-bc.f90 unsupported on Darwin (#171693)
It fails with the following error:
```
RUN: at line 23
/Users/leandro.lupori/home/git/flang/build/bin/flang -flto /Users/leandro.lupori/home/git/flang/llvm-project/flang/test/Driver/lto-bc.f90 -c -o - | llvm-dis -o - | /Users/leandro.lupori/home/git/flang/build/bin/FileCheck /Users/leandro.lupori/home/git/flang/llvm-project/flang/test/Driver/lto-bc.f90 --check-prefix=FULL
.---command stderr------------
| /Users/leandro.lupori/home/git/flang/llvm-project/flang/test/Driver/lto-bc.f90:27:9: error: FULL: expected string not found in input
| ! FULL: !{{.*}} = !{i32 1, !"ThinLTO", i32 0}
| ^
| ...
| Input was:
| <<<<<<
| ...
| 24: !llvm.ident = !{!0}
| check:27'0 ~~~~~~~~~~~~~~~~~~~~
| 25: !llvm.module.flags = !{!1, !2}
| check:27'0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 26:
| check:27'0 ~
| 27: !0 = !{!"flang version 22.0.0 (git@github.com:llvm/llvm-project.git dde1990b01fa13087b0eab77579f528329555968)"}
| check:27'0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 28: !1 = !{i32 2, !"Debug Info Version", i32 3}
| check:27'0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 29: !2 = !{i32 8, !"PIC Level", i32 2}
| check:27'0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| check:27'1 ? possible intended match
| >>>>>>
`-----------------------------
```
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