rsmith added a comment.

I really don't like the command-line interface you're proposing here. It seems 
like it will be extremely confusing what something like `-fmodules 
-fexclusive-builtin-modules` means, for instance (usually, later `-f` flags 
override earlier ones, so does this *turn off* modules for everything other 
than builtin headers?). Instead, we should use a command-line interface that 
more naturally / obviously composes with other flags.

It seems like the behavior you want here is precisely:

  `-fmodules -fno-implicit-module-maps -fmodule-map-file=<resource 

(The `-fmodules-validate-system-headers` part would only make sense for people 
using clang directly from their build area; installed / released versions of 
clang should not need this. Adding it makes your flag compose poorly with other 
uses of modules.)

I'd be happy with you adding a flag that is equivalent to 
`-fmodule-map-file=<resource dir>/include/module.modulemap`.

Also, it seems unlikely to me that we would ever turn this on by default. It 
will cause havoc for distributed builds, especially ones that run compilation 
actions in 'clean' environments, as (for instance) it will cause the complete 
set of intrinsics headers to be compiled into a module on every compile. So if 
this is your proposed solution for slow compiles using intrinsics headers, you 
should be aware that this is not a complete solution to that problem.

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