Attempt to fix PR20884

This fixes the generation of broken LLVMExports.cmake file by
the Autoconf/Makefile build system when --enable-shared is passed to
configure.

When --enable_shared is passed the Makefile.rules does not set the
LLVMConfigLibs variable which cmake/modules/Makefile previously relied
on. Now it runs the llvm-config command itself to get the library names.

This still isn't perfect because the generated LLVM targets refer to the
static libraries and not the shared library but that is much larger
problem to fix.

llvm-svn: 217484