[Tooling] Fix broken dependency for shared build

Summary:
There virtual destructor can't be found and cause a compilation error
on a shared build.

To repro: [Release + Shared]
```
-DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=ON
```

Which produce this error:
```
tools/clang/unittests/Tooling/CMakeFiles/ToolingTests.dir/ToolingTest.cpp.o: In function `clang::tooling::newFrontendActionFactory_CreatesFrontendActionFactoryFromType_Test::TestBody()':
ToolingTest.cpp:(.text._ZN5clang7tooling66newFrontendActionFactory_CreatesFrontendActionFactoryFromType_Test8TestBodyEv+0x49): undefined reference to `clang::SyntaxOnlyAction::~SyntaxOnlyAction()'
```

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20218

llvm-svn: 269334