Rename llvm library from libLLVM-X.Y to libLLVM-X

Summary:
As we are only doing X.0.Z releases (not using the minor version), there is no need to keep -X.Y in the version.

Like patch https://reviews.llvm.org/D41808, I propose that we rename libLLVM-7.0svn.so to libLLVM-7svn.so 
This patch will also rename downstream libraries like liblldb-7.0 to liblldb-7

Reviewers: axw, beanz, dim, hans

Reviewed By: dim, hans

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D41869

llvm-svn: 328768