msan_builder: Inject MSan flags in wrapper if they don't exist.

Some packages build test binaries that don't use DEB_C(XX)FLAGS and fail
during linking.

infra/base-images/msan-builder/compiler_wrapper.py

@@ -20,6 +20,8 @@ import os
 import subprocess
 import sys
 
+import msan_build
+
 
 def Is32Bit(args):
   """Return whether or not we're 32-bit."""
@@ -72,6 +74,10 @@ def GetCompilerArgs(args):
       '-w',
   ])
 
+  if '-fsanitize=memory' not in args:
+    # If MSan flags weren't added for some reason, add them here.
+    compiler_args.extend(msan_build.INJECTED_ARGS)
+
   return compiler_args