scan-build-py: fix multiprocessing error

Recent versions of python3's multiprocessing module will blow up with
a Runtime error from this code, saying:

  An attempt has been made to start a new process before the
  current process has finished its bootstrapping phase

This is becuae the wrappers in bin/ are not using the  `__name__ == "__main__"`   idiom correctly.

Reviewed By: ldionne

Differential Revision: https://reviews.llvm.org/D87051

clang/tools/scan-build-py/bin/analyze-build

@@ -5,12 +5,13 @@
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 import multiprocessing
-multiprocessing.freeze_support()
-
 import sys
 import os.path
 this_dir = os.path.dirname(os.path.realpath(__file__))
 sys.path.append(os.path.dirname(this_dir))
 
 from libscanbuild.analyze import analyze_build
-sys.exit(analyze_build())
+
+if __name__ == '__main__':
+    multiprocessing.freeze_support()
+    sys.exit(analyze_build())

clang/tools/scan-build-py/bin/intercept-build

@@ -5,12 +5,13 @@
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 import multiprocessing
-multiprocessing.freeze_support()
-
 import sys
 import os.path
 this_dir = os.path.dirname(os.path.realpath(__file__))
 sys.path.append(os.path.dirname(this_dir))
 
 from libscanbuild.intercept import intercept_build
-sys.exit(intercept_build())
+
+if __name__ == '__main__':
+    multiprocessing.freeze_support()
+    sys.exit(intercept_build())

clang/tools/scan-build-py/bin/scan-build

@@ -5,12 +5,13 @@
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 import multiprocessing
-multiprocessing.freeze_support()
-
 import sys
 import os.path
 this_dir = os.path.dirname(os.path.realpath(__file__))
 sys.path.append(os.path.dirname(this_dir))
 
 from libscanbuild.analyze import scan_build
-sys.exit(scan_build())
+
+if __name__ == '__main__':
+    multiprocessing.freeze_support()
+    sys.exit(scan_build())