[lit] Add a lit.llvm module that all test suites can use.

To further reduce duplicate code, this patch introduces a module
that configs can simply import and get access to a lot of useful
functionality such as setting up paths, adding features that are
useful across all projects, and other utility-type functions.

For now this only updates llvm's suite to use this new library,
but subsequent patches will update other projects.

Differential Revision: https://reviews.llvm.org/D37778

llvm-svn: 313325

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