Export the required symbol from DynamicLibraryTests

Running unittests/Support/DynamicLibrary/DynamicLibraryTests fails
when LLVM is configured with -DLLVM_EXPORT_SYMBOLS_FOR_PLUGINS=ON, because
the test's version script only contains symbols extracted from the static libraries,
that the test links with, but not those from the main object/executable itself.

The patch moves the one symbol, needed by the test, to a static library.

Fixes https://bugs.llvm.org/show_bug.cgi?id=32893

Patch by Momchil Velikov.

Differential Revision: https://reviews.llvm.org/D33789

llvm-svn: 305181