[cxxmodules] Disable module file validation.

We need to disable the validation of PCM files because rootcling
serializes what it sees. For instance, we even serialize in the PCM
file some temporary lookup buffers and the contents of the module maps.

We disable the PCH validation but that is not sufficient for PCM. The
TCling code `fInterpreter->getCI()->getPreprocessorOpts().DisablePCHValidation = true`
is not enough because we rely more on the clang driver to setup correctly
our modules-aware interpreter.

This patch just uses the correct flag when setting up cling.

A proper fix to this issue is thoroughly described in RE-0003 (rootcling
refactor proposal).

core/metacling/src/TCling.cxx

@@ -1159,6 +1159,7 @@ static void LoadCoreModules(cling::Interpreter &interp)
    // them. This will be wrapped into an `import` declaration by clang, so we
    // only make those modules available, not actually textually include those
    // headers.
+   // FIXME: Use clang::ASTReader::makeModuleVisible.
    std::stringstream declarations;
    for (StringRef H : moduleHeaders) {
       declarations << "#include \"" << H.str() << "\"\n";

interpreter/cling/lib/Interpreter/CIFactory.cpp

@@ -825,7 +825,16 @@ static void stringifyPreprocSetting(PreprocessorOptions& PPOpts,
       // is a ROOT-specific issue tracked by ROOT-9088.
       // FIXME: Remove after merging ROOT's PR1306.
       argvCompile.push_back("-Wno-modules-import-nested-redundant");
-
+      // FIXME: We get an error "'cling/module.modulemap' from the precompiled
+      //  header has been overridden". This comes from a bug that rootcling
+      // introduces by adding a lot of garbage in the PCH/PCM files because it
+      // essentially serializes its current state of the AST. That usually
+      // includes a few memory buffers which override their own contents.
+      // We know how to remove this: just implement a callback in clang
+      // which calls back the interpreter when a module file is built. This is
+      // a lot of work as it needs fixing rootcling. See RE-0003.
+      argvCompile.push_back("-Xclang");
+      argvCompile.push_back("-fno-validate-pch");
     }
 
     if (!COpts.Language) {