build-script: support CMake install steps.

Currently, bots that perfrom an installation will build LLDB once using CMake
because that is the default and then again using Xcode because the CMake
install step is not implemented by build-script. This patch adds CMake support
by calling the generic cmake install-lldb step.

<rdar://problem/51317901>

Author: adrian-prantl

utils/build-script-impl

@@ -3537,24 +3537,27 @@ for host in "${ALL_HOSTS[@]}"; do
                     echo "--install-destdir is required to install products."
                     exit 1
                 fi
-
-                case ${host} in
-                    linux-*)
+                if [[ "$using_xcodebuild" == "TRUE" ]] ; then
+                    case ${host} in
+                        linux-*)
                         ;;
-                    freebsd-*)
+                        freebsd-*)
                         ;;
-                    cygwin-*)
+                        cygwin-*)
                         ;;
-                    haiku-*)
+                        haiku-*)
                         ;;
-                    macosx-*)
-                        set_lldb_xcodebuild_options
-                        set_lldb_build_mode
-                        with_pushd ${LLDB_SOURCE_DIR} \
-                            call xcodebuild -target toolchain -configuration ${LLDB_BUILD_MODE} install ${lldb_xcodebuild_options[@]} DSTROOT="${host_install_destdir}" LLDB_TOOLCHAIN_PREFIX="${TOOLCHAIN_PREFIX}"
-                        continue
+                        macosx-*)
+                            set_lldb_xcodebuild_options
+                            set_lldb_build_mode
+                            with_pushd ${LLDB_SOURCE_DIR} \
+                                       call xcodebuild -target toolchain -configuration ${LLDB_BUILD_MODE} install ${lldb_xcodebuild_options[@]} DSTROOT="${host_install_destdir}" LLDB_TOOLCHAIN_PREFIX="${TOOLCHAIN_PREFIX}"
+                            continue
                         ;;
-                esac
+                    esac
+                else
+                    INSTALL_TARGETS="install-lldb"
+                fi
                 ;;
             swiftpm)
                 if [[ -z "${INSTALL_SWIFTPM}" ]] ; then