[Modules] Add 'no_undeclared_includes' module map attribute

The 'no_undeclared_includes' attribute should be used in a module to
tell that only non-modular headers and headers from used modules are

[Modules] Add 'no_undeclared_includes' module map attribute

The 'no_undeclared_includes' attribute should be used in a module to
tell that only non-modular headers and headers from used modules are
accepted.

The main motivation behind this is to prevent dep cycles between system
libraries (such as darwin) and libc++.

Patch by Richard Smith!

llvm-svn: 284797

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