Add check for Xcode version compatibility

This adds a check validating the current Xcode version is supported for
building the current sha. This makes us fail fast in the case that
you've selected too new or too old of an Xcode version that might
otherwise fail very late in the build process. You can also set
SKIP_XCODE_VERSION_CHECK to anything to skip this.

Author: keith

utils/build-script

@@ -53,6 +53,15 @@ from swift_build_support.swift_build_support.toolchain import host_toolchain
 # TODO: Remove this constant, it's really not helpful.
 HOME = os.environ.get("HOME", "/")
 
+_SUPPORTED_XCODE_BUILDS = (
+    "12A8169g", # 12.0b3
+    "12A8179i", # 12.0b4
+    "12A8189h", # 12.0b5
+    "12A8189n", # 12.0b6
+    "12A7208", # 12.0GM1
+    "12A7209", # 12.0GM2
+    "12A7300", # 12.0.1
+)
 
 # -----------------------------------------------------------------------------
 # Helpers
@@ -147,6 +156,22 @@ def print_xcodebuild_versions(file=sys.stdout):
     print(fmt.format(version=version, sdks=sdks), file=file)
     file.flush()
 
+def validate_xcode_compatibility():
+    if sys.platform != 'darwin':
+        return
+
+    if os.getenv("SKIP_XCODE_VERSION_CHECK"):
+        print("note: skipping Xcode version check")
+        return
+
+    version = shell.capture(
+        ['xcodebuild', '-version'], dry_run=False, echo=False).strip()
+    if not version.endswith(_SUPPORTED_XCODE_BUILDS):
+        raise SystemExit(
+            "error: using unsupported Xcode version:\n\n{}\n\nInstall one of: {}".format(
+                version, ", ".join(_SUPPORTED_XCODE_BUILDS)
+            )
+        )
 
 def tar(source, destination):
     """
@@ -1335,6 +1360,8 @@ def main():
             "\' does not exist " +
             "(forgot to set $SWIFT_SOURCE_ROOT environment variable?)")
 
+    validate_xcode_compatibility()
+
     # Determine if we are invoked in the preset mode and dispatch accordingly.
     if any([(opt.startswith("--preset") or opt == "--show-presets")
             for opt in sys.argv[1:]]):