[llvm] Automatically exporting symbols for C API on MSVC for LLVM.dll

[Remi Gillig via llvm-commits]

Hello,

I needed to export symbols for the C API into LLVM.dll. The existing option
LLVM_BUILD_LLVM_DYLIB=ON didn't work as expected. The comment the top of
`tools/llvm-shlib/CMakeLists.txt` seemed to imply that it should export the
C API. However, that wasn't the case on MSVC.

The attached patch fixes that, hopefully in the least intrusive way. I
tested it with CMake 3.5.2 and VS 2015.

This is my first patch to LLVM, I tried to follow the steps of the
developer policy document. Let me know if there's anything else I need to
do or if I did anything wrong.

Thanks,
Remi Gillig.
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