Fix issues building libraries as more than one type with Xcode

Summary:
CMake+Xcode doesn't seem to handle targets that only have object
sources. This patch works around that limitation by adding a dummy
soruce file to any library target that is generated by llvm_add_library
when object libraries are generated.

Object libraries are generated whenever llvm_add_library is passed more
than one library type, which is now the default case for clang static
libraries (which generate STATIC and OBJECT libraries).

Reviewers: zturner, compnerd, joanlluch

Reviewed By: joanlluch

Subscribers: joanlluch, xbolva00, mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64300

llvm-svn: 365365