[C++20] Diagnose invalid and reserved module names

[module.unit]p1 specifies that module and import are invalid components
of a module name, that module names cannot contain reserved
identifiers, and that std followed by zero or more digits is reserved.

The first issue (module and import pseudo-keywords) requires a
diagnostic, the second issue (use of reserved identifiers) does not
require a diagnostic. We diagnose both the same -- the code is ill-
formed unless the module declaration is in a system "header". This
allows STL implementations to use the reserved module names while
preventing users from stealing them out from under us.

Differential Revision: https://reviews.llvm.org/D136953