libclang.so: Make SONAME the same as LLVM version

This partially reverts c7b3a910.  Having
libclang.so with a different SONAME than the other LLVM libraries was
causing a lot of confusion for users.  Also, this change did not really
acheive it's purpose of allowing apps to use newer versions of
libclang.so without rebuilding, because a new version of libclang.so
requires a new version of libLLVM.so, which does not have a stable ABI.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D129160