Custom LLVM Versions for VS2019+

What does this PR do?

Adds an API to Visual Studio exporter for specifying custom LLVM versions and installation directories.

How does this PR change Premake's behavior?

No breaking changes.

Anything else we should know?

Closes #1864

Did you check all the boxes?

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

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premake / premake-core