[clang] Do not override libclang.so's SOVERSION if CLANG_FORCE_MATCHING_LIBCLANG_SOVERSION

Instead of setting libclang.so's SOVERSION to CLANG_MAJOR_VERSION
when CLANG_FORCE_MATCHING_LIBCLANG_SOVERSION is enabled, do not override
it and let LLVM set the correct SOVERSION.  This fixes libclang.so
missing LLVM_VERSION_SUFFIX, and harmonizes the SOVERSION with other
shared libraries.

Differential Revision: https://reviews.llvm.org/D135701