[cmake] Expose the dependencies of ExecutionEngine as PUBLIC

Expose the dependencies of LLVMExecutionEngine library as PUBLIC rather
than PRIVATE when building a shared library. This is necessary because
the library is not contained but exposes API of other LLVM libraries via
its headers.

This causes other libraries to fail to link if the linker verifies for
correctness of -l flags (i.e. fails on indirect dependencies). This e.g.
happens when building LLDB against shared LLVM:

  lib64/liblldbExpression.a(IRExecutionUnit.cpp.o):(.data.rel.ro._ZTIN4llvm18MCJITMemoryManagerE[_ZTIN4llvm18MCJITMemoryManagerE]+0x10): undefined reference to `typeinfo for llvm::RuntimeDyld::MemoryManager'
  lib64/liblldbExpression.a(IRExecutionUnit.cpp.o):(.data.rel.ro._ZTVN4llvm18MCJITMemoryManagerE[_ZTVN4llvm18MCJITMemoryManagerE]+0x60): undefined reference to `llvm::RuntimeDyld::MemoryManager::anchor()'
  lib64/liblldbExpression.a(IRExecutionUnit.cpp.o):(.data.rel.ro._ZTVN12lldb_private15IRExecutionUnit13MemoryManagerE[_ZTVN12lldb_private15IRExecutionUnit13MemoryManagerE]+0x48): undefined reference to `llvm::RTDyldMemoryManager::deregisterEHFrames()'
  lib64/liblldbExpression.a(IRExecutionUnit.cpp.o):(.data.rel.ro._ZTVN12lldb_private15IRExecutionUnit13MemoryManagerE[_ZTVN12lldb_private15IRExecutionUnit13MemoryManagerE]+0x60): undefined reference to `llvm::RuntimeDyld::MemoryManager::anchor()'
  lib64/liblldbExpression.a(IRExecutionUnit.cpp.o):(.data.rel.ro._ZTVN12lldb_private15IRExecutionUnit13MemoryManagerE[_ZTVN12lldb_private15IRExecutionUnit13MemoryManagerE]+0xd0): undefined reference to `llvm::JITSymbolResolver::anchor()'
  collect2: error: ld returned 1 exit status

Declaring the dependencies as PUBLIC guarantees that any package using
the ExecutionEngine library will also get explicit -l flags for
the dependent libraries guaranteeing that the symbols exposed in headers
could be resolved.

Patch originally written by NAKAMURA Takumi.

Differential Revision: https://reviews.llvm.org/D36211

llvm-svn: 310712