Rename clang link from clang-X.Y to clang-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.
So, instead, I propose the following:
Instead of having clang-7.0 in bin/, we will have clang-7

Since also matches was gcc is doing.

Reviewers: tstellar, dlj, dim, hans

Reviewed By: dim, hans

Subscribers: dim, mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D41808

llvm-svn: 328769