[cxxmodules] Fall back to Header parsing on demand if we have no module. #2721
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We assumed that we will always have module file and unconditionally disable
header parsing on demand. However, the major use-case is gradual migration
to modules. In this scenario (tested by root-meta-fwdDecls-fwdDeclarations),
we have a dictionary which has no module file and still relies on the old
behavior. This can realistically happen when users gradually migrate to
modules. For example, we have modules-aware ROOT and untouched third party
code.
This patch enables header parsing on demand when we have no module file
available.