[clang] Add tests for CWG issues about language linkage (#107019)

This patch covers Core issues about language linkage during declaration
matching resolved in
[P1787R6](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1787r6.html),
namely [CWG563](https://cplusplus.github.io/CWG/issues/563.html) and
[CWG1818](https://cplusplus.github.io/CWG/issues/1818.html).

[CWG563](https://cplusplus.github.io/CWG/issues/563.html) "Linkage
specification for objects"
-----------

[P1787R6](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1787r6.html):
> [CWG563](https://cplusplus.github.io/CWG/issues/563.html) is resolved
by simplifications that follow its suggestions.

Wording ([[dcl.link]/5](https://eel.is/c++draft/dcl.link#5)):
> In a
[linkage-specification](https://eel.is/c++draft/dcl.link#nt:linkage-specification),
the specified language linkage applies to the function types of all
function declarators and to all functions and variables whose names have
external linkage[.](https://eel.is/c++draft/dcl.link#5.sentence-5)

Now the wording clearly says that linkage-specification applies to
variables with external linkage.

[CWG1818](https://cplusplus.github.io/CWG/issues/1818.html) "Visibility
and inherited language linkage"
------------

[P1787R6](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1787r6.html):
>
[CWG386](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#386),
[CWG1839](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1839),
[CWG1818](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1818),
[CWG2058](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#2058),
[CWG1900](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1900),
and Richard’s observation in [“are non-type names ignored in a
class-head-name or
enum-head-name?”](http://lists.isocpp.org/core/2017/01/1604.php) are
resolved by describing the limited lookup that occurs for a
declarator-id, including the changes in Richard’s [proposed resolution
for
CWG1839](http://wiki.edg.com/pub/Wg21cologne2019/CoreWorkingGroup/cwg1839.html)
(which also resolves CWG1818 and what of CWG2058 was not resolved along
with CWG2059) and rejecting the example from
[CWG1477](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1477).

Wording ([[dcl.link]/6](https://eel.is/c++draft/dcl.link#6)):
> A redeclaration of an entity without a linkage specification inherits
the language linkage of the entity and (if applicable) its
type[.](https://eel.is/c++draft/dcl.link#6.sentence-2).

Answer to the question in the example is `extern "C"`, and not linkage
mismatch. Further analysis of the example is provided as inline comments
in the test itself. Note that https://eel.is/c++draft/dcl.link#7 does
NOT apply in this example, as it's focused squarely at declarations that
are already known to have C language linkage, and declarations of
variables in the global scope.