Fix DynamicLibraryTests build on Windows when LLVM_EXPORT_SYMBOLS_FOR_PLUGINS is ON

extract_symbols.py (introduced in D18826) expects all of its library arguments
to be in the same directory - typically <config>/lib. DynamicLibraryLib.lib is
instead to be found in lib/<config>.
This patch intended to make DynamicLibraryLib.lib be created in <config>/lib
alongside most of the other libraries.

I previously tried passing absolute paths to extract_symbols.py but this
generated command lines that were too long for Visual Studio 2015: D54587

Differential Revision: https://reviews.llvm.org/D54701

llvm-svn: 347764