[build] Verify a host is supported when setting build options

By verifying that the host is supported before doing any operations, we can
assume that the host value is properly formed, meaning we can simplify the logic
to set values based on the host. Follow-up commits will do exactly that.

Author: bulbazord

utils/build-script-impl

@@ -397,6 +397,40 @@ function should_execute_host_actions_for_phase() {
     fi
 }
 
+function verify_host_is_supported() {
+    local host="$1"
+    case ${host} in
+      freebsd-x86_64            \
+      | cygwin-x86_64           \
+      | haiku-x86_64            \
+      | linux-x86_64            \
+      | linux-i686              \
+      | linux-armv6             \
+      | linux-armv7             \
+      | linux-aarch64           \
+      | linux-powerpc64         \
+      | linux-powerpc64le       \
+      | linux-s390x             \
+      | macosx-x86_64           \
+      | iphonesimulator-i386    \
+      | iphonesimulator-x86_64  \
+      | iphoneos-armv7          \
+      | iphoneos-armv7s         \
+      | iphoneos-arm64          \
+      | appletvsimulator-x86_64 \
+      | appletvos-arm64         \
+      | watchsimulator-i386     \
+      | watchos-armv7k          \
+      | android-armv7           \
+      | android-aarch64)
+        ;;
+      *)
+        echo "Unknown host tools target: ${host}"
+        exit 1
+        ;;
+    esac
+}
+
 function set_build_options_for_host() {
     llvm_cmake_options=()
     swift_cmake_options=()
@@ -411,6 +445,8 @@ function set_build_options_for_host() {
     # Hosts which can be cross-compiled must specify:
     # SWIFT_HOST_TRIPLE and llvm_target_arch (as well as usual HOST_VARIANT flags)
 
+    verify_host_is_supported $host
+
     case ${host} in
         freebsd-x86_64)
             SWIFT_HOST_VARIANT="freebsd"
@@ -555,10 +591,6 @@ function set_build_options_for_host() {
                     SWIFT_HOST_VARIANT_ARCH="armv7k"
                     cmake_osx_deployment_target=""
                     ;;
-                *)
-                    echo "Unknown host for swift tools: ${host}"
-                    exit 1
-                    ;;
             esac
 
             if [[ "${DARWIN_SDK_DEPLOYMENT_TARGETS}" != "" ]]; then
@@ -638,10 +670,6 @@ function set_build_options_for_host() {
                     ;;
                 esac
         ;;
-        *)
-            echo "Unknown host tools target: ${host}"
-            exit 1
-            ;;
     esac