Add hidden --dump-source-file-imports flag

This flag dumps all imports for each SourceFile after it's gone through
import resolution. It is only intended for testing purposes.
There are other ways to print imports, but they don't correspond 1:1 to
the imports actually resolved, which is a bit problematic when testing
implicit clang module imports.

lib/Frontend/CompilerInvocation.cpp

@@ -1730,6 +1730,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.DumpMacroExpansions = Args.hasArg(
       OPT_dump_macro_expansions);
 
+  Opts.DumpSourceFileImports = Args.hasArg(
+      OPT_dump_source_file_imports);
+
   if (const Arg *A = Args.getLastArg(OPT_debug_requirement_machine))
     Opts.DebugRequirementMachine = A->getValue();