[CMake] Get rid of LLVM_DYLIB_EXPORT_ALL, and make it the default, add libLLVM-C on darwin to cover the C API needs.

Summary:
We've had a lot of discussion in the past about the meaningful and useful default behaviors for the llvm-shlib tool. The original implementation was heavily geared toward Apple's use, and I think that was wrong. This patch seeks to correct that.

I've removed the LLVM_DYLIB_EXPORT_ALL variable and made libLLVM export everything by default.

I've also added a new target that is only built on Darwin for libLLVM-C as a library that re-exports the LLVM-C API. This library is not built on Linux because ELF doesn't support re-export libraries in the same way MachO does.

Reviewers: chapuni, resistor, bogner, axw

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13842

llvm-svn: 251411