[build-script-impl][lto] When compiling swift/llvm with lto, pass LLVM_PARALLEL_LINK_JOBS to swift as well as llvm.

The reason to do this is the same reason that we already pass in
-DLLVM_ENABLE_LTO=YES to swift, to ensure that parts of our codebase that use
the llvm build machinery (e.g. the unittests) do not use too many linker jobs
given the memory constraints of a given machine.

rdar://24717107

Author: gottesmm

utils/build-script-impl

@@ -649,6 +649,15 @@ function set_build_options_for_host() {
                         "-DCMAKE_CXX_STANDARD_COMPUTED_DEFAULT=AppleClang"
                     )
                 fi
+
+                # We need to also pass in this flag so that parts of LLVM's
+                # build system that we use (for instance when compiling
+                # unittests) do not use too many threads when we are using lto.
+                if [[ $(true_false "${LLVM_ENABLE_LTO}") == "TRUE" ]]; then
+                    swift_cmake_options+=(
+                        "-DLLVM_PARALLEL_LINK_JOBS=${LLVM_NUM_PARALLEL_LTO_LINK_JOBS}"
+                    )
+                fi
                 swift_cmake_options+=(
                     "-DSWIFT_PARALLEL_LINK_JOBS=$(num_swift_parallel_lto_link_jobs)"
                 )