Fix unresolved external symbol errors on Windows

Resolves root-project/cling#399
Fix the following linker errors:
253>cling.exp : error LNK2001: unresolved external symbol "public: static class std::basic_string<char,struct std::char_traits,class std::allocator > __cdecl clang::QualType::getAsString(class clang::Type const *,class clang::Qualifiers)" (?getAsString@QualType@clang@@sa?AV?$basic_string@DU?$char_traits@D@std@@v?$allocator@D@2@@std@@PEBVType@2@VQualifiers@2@@z)
253>cling.exp : error LNK2001: unresolved external symbol "private: virtual void __cdecl llvm::raw_ostream::handle(void)" (?handle@raw_ostream@llvm@@EEAAXXZ)
253>D:\work\libs\cling\src\build\Release\bin\cling.exe : fatal error LNK1120: 2 unresolved externals