[llvm] Add CMake flag to compile out the telemetry framework

[JDevlieghere]

Add a CMake flag (LLVM_BUILD_TELEMETRY) to disable building the telemetry framework. The flag being enabled does not mean that telemetry is being collected, it merely means we're building the generic telemetry framework. Hence the flag is enabled by default.

Motivated by this Discourse thread: https://discourse.llvm.org/t/how-to-disable-building-llvm-clang-telemetry/84305

[tstellar]

Does this change need to go to the release//20.x branch?

[JDevlieghere]

Does this change need to go to the release//20.x branch?

Yup, I'll nominate it when the PR is approved.

[jh7370]

LGTM, but I'd like @oontvoo to confirm before this gets merged, if possible.

[oontvoo]

Quick question: why isn't using the existing option to disable the framework sufficient?

Otherwise, LGTM.

[JDevlieghere]

Quick question: why isn't using the existing option to disable the framework sufficient?

By the existing option, I assume you're referring to the Telemetry::Config struct? I can't speak for the original author on Discourse, but I would assume it's because it's much easier to verify statically that you don't have any telemetry code when it has been compiled out (e.g. you can run strings on the binary) compared to dynamically where you have to prove that code is never called.

[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/2223

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-12124-38-87.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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