[CMake] Add libunwind when 'all' is being passed as LLVM_ENABLE_PROJECTS

[ldionne]

@zturner Sorry if you're the wrong person to look at this, but you're on the reviewer list for the last change to this line. I'll find someone else if you're the wrong person.

[ldionne]

Got LGTM by @zturner on the mailing list.

llvm/CMakeLists.txt

if(DYLD_LIBRARY_PATH)
     endforeach()
   endif()
 endif()
 
 # Side-by-side subprojects layout: automatically set the
 # LLVM_EXTERNAL_${project}_SOURCE_DIR using LLVM_ALL_PROJECTS
 # This allows an easy way of setting up a build directory for llvm and another
 # one for llvm+clang+... using the same sources.
-set(LLVM_ALL_PROJECTS "clang;libcxx;libcxxabi;lldb;compiler-rt;lld;polly;debuginfo-tests")
+set(LLVM_ALL_PROJECTS "clang;libcxx;libcxxabi;libunwind;lldb;compiler-rt;lld;polly;debuginfo-tests")
 set(LLVM_ENABLE_PROJECTS "" CACHE STRING
   "Semicolon-separated list of projects to build (${LLVM_ALL_PROJECTS}), or \"all\".")
 if( LLVM_ENABLE_PROJECTS STREQUAL "all" )
   set( LLVM_ENABLE_PROJECTS ${LLVM_ALL_PROJECTS})
 endif()
 foreach(proj ${LLVM_ENABLE_PROJECTS})
   set(PROJ_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")
   if(NOT EXISTS "${PROJ_DIR}" OR NOT IS_DIRECTORY "${PROJ_DIR}")