[flang] Move LLVM_ENABLE_RUNTIMES definition above uses

[DavidTruby]

This fixes an issue where flang-rt was not getting automatically added
to LLVM_ENABLE_RUNTIMES because LLVM_ENABLE_RUNTIMES had not been
defined yet.

[dpalermo]

LGTM. Thanks!

[clementval]

I had the same issue yesterday. Thanks for fixing this.

[Meinersbur]

Thanks for the fix

[llvm-ci]

LLVM Buildbot has detected a new failure on builder lld-x86_64-win running on as-worker-93 while building llvm at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/146/builds/2831

Here is the relevant piece of the build log for the reference

Step 7 (test-build-unified-tree-check-all) failure: test (failure) ******************** TEST 'LLVM-Unit :: Support/./SupportTests.exe/90/95' FAILED ******************** Script(shard): -- GTEST_OUTPUT=json:C:\a\lld-x86_64-win\build\unittests\Support\.\SupportTests.exe-LLVM-Unit-20552-90-95.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=95 GTEST_SHARD_INDEX=90 C:\a\lld-x86_64-win\build\unittests\Support\.\SupportTests.exe -- Script: -- C:\a\lld-x86_64-win\build\unittests\Support\.\SupportTests.exe --gtest_filter=ProgramEnvTest.CreateProcessLongPath -- C:\a\lld-x86_64-win\llvm-project\llvm\unittests\Support\ProgramTest.cpp(160): error: Expected equality of these values: 0 RC Which is: -2 C:\a\lld-x86_64-win\llvm-project\llvm\unittests\Support\ProgramTest.cpp(163): error: fs::remove(Twine(LongPath)): did not return errc::success. error number: 13 error message: permission denied C:\a\lld-x86_64-win\llvm-project\llvm\unittests\Support\ProgramTest.cpp:160 Expected equality of these values: 0 RC Which is: -2 C:\a\lld-x86_64-win\llvm-project\llvm\unittests\Support\ProgramTest.cpp:163 fs::remove(Twine(LongPath)): did not return errc::success. error number: 13 error message: permission denied ********************