tools/llvm-config: improve shared library support

Summary:
r252532 added support for reporting the monolithic library
when LLVM_BUILD_LLVM_DYLIB is used. This would only be done
if the individual components were not found, and the dynamic
library is found.

This diff extends this as follows:
 - If LLVM_LINK_LLVM_DYLIB is set, then prefer the shared
   library, even if all component libraries exist.
 - Two flags, --link-shared and --link-static are introduced
   to provide explicit guidance. If --link-shared is passed
   and the shared library does not exist, an error results.

Additionally, changed the expected shared library names from
(e.g.) LLVM-3.8.0 to LLVM-3.8. The former exists only in an
installation (and then only in CMake builds I think?), and not
in the build tree; this breaks usage of llvm-config during
builds, e.g. by llvm-go.

Reviewers: DiamondLovesYou, beanz

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15033

llvm-svn: 257003