clang Use llvm-ar linker when LTO flag is set.

[JoelLinn]

What does this PR do?

Changes the static linker from ar to llvm-ar when the clang toolset is used and the LinkTimeOptimization flag is set.
Although many times GNU ar will also work, for some clang features like -flto (link-time optimisation, I only came across this one but could be more) llvm-ar is needed.

How does this PR change Premake's behavior?

It should not break or change build-file output for existing projects (that may not have llvm-ar installed) since llvm-ar is only conditionally used and still defaults to ar.

Anything else we should know?

Before this change, it was not possible to use the LinkTimeOptimization flag with the clang toolset unless using the "AR=llvm-ar" environment variable.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits

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[samsinsane]

Please fill in the template, it's there for a reason.

[JoelLinn]

Please fill in the template, it's there for a reason.

please reopen

[tritao]

It seems a bit inconsistent that the tool will be different depending on if LTO is enabled or not.

Is there any reason not to just always use llvm-ar when using the clang toolset?

[JoelLinn]

Is there any reason not to just always use llvm-ar when using the clang toolset?

• It may break existing projects since usually llvm packages are not required by clang-packages. (llvm-ar is not installed)
• Afaik (if that counts anything here) cmake also uses plain ar first in its makefiles unless certain features are enabled, only then llvm-ar, llvm-ranlib and friends are emitted. I never looked into when exactly they switched, just noticed it in the makefiles

[tritao]

Makes sense, thanks.