[Polly][CMake] Fix exports

[nikic]

If Polly is built with LLVM_POLLY_LINK_INTO_TOOLS=ON (the default for monorepo builds), then Polly will become a dependency of the LLVMExtensions component, which is part of LLVMExports. As such, all the Polly libraries also have to be part of LLVMExports.

However, if Polly is built with LLVM_POLLY_LINK_INTO_TOOLS=OFF, we also end up adding Polly libraries to LLVMExports. This is undesirable, as it adds a hard dependency from llvm on polly.

Fix this by only exporting polly libraries from LLVMExports if LLVM_POLLY_LINK_INTO_TOOLS is enabled.

[nikic]

Also have to declare LLVM_POLLY_LINK_INTO_TOOLS earlier, because we use it before llvm_add_pass_plugin is called. Didn't notice this during testing because I was passing it explicitly...

[nikic]

Ping :)

[sebpop]

The change looks good to me.

[llvm-ci]

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

Full details are available at: https://lab.llvm.org/buildbot/#/builders/146/builds/2122

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/38/87' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:C:\a\lld-x86_64-win\build\unittests\Support\.\SupportTests.exe-LLVM-Unit-18384-38-87.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=87 GTEST_SHARD_INDEX=38 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




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