Support of LLVM

Version 0.3 was released:

1. Task llvmCleanCache was added. The task performs clean of local user cache with LLVM downloads.

2. A shared linkage was implemented. Only version LLVM 9.0.0 for GCC, MinGW-W64, and MSVC is supported at the moment.

   llvm {
        version = '9.0.0'
        linkage = Linkage.SHARED
    }