This PR is 2nd part of
[P1857R3](https://github.com/llvm/llvm-project/pull/107168)
implementation, and mainly implement the restriction `A module directive
may only appear as the first preprocessing tokens in a file (excluding
the global module fragment.)`:
[cpp.pre](https://eel.is/c++draft/cpp.pre):
```
module-file:
pp-global-module-fragment[opt] pp-module group[opt] pp-private-module-fragment[opt]
```
We also refine tests use `split-file` instead of conditional macro.
Signed-off-by: yronglin <yronglin777@gmail.com>
[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
