[clang] Add tests for Core issues about friend templates (#94288)

This patch covers the following Core issues:
[CWG1862](https://cplusplus.github.io/CWG/issues/1862.html) "Determining
“corresponding members” for friendship"
[CWG1918](https://cplusplus.github.io/CWG/issues/1918.html) "`friend`
templates with dependent scopes"
[CWG1945](https://cplusplus.github.io/CWG/issues/1945.html) "Friend
declarations naming members of class templates in non-templates"

All of them were resolved by CWG1862, where the current wording of
[[temp.friend] p5](https://eel.is/c++draft/temp.friend#5) comes from:
> A template friend declaration may declare a member of a dependent type
to be a friend[.](https://eel.is/c++draft/temp.friend#5.sentence-1) The
friend declaration shall declare a function or specify a type with an
[elaborated-type-specifier](https://eel.is/c++draft/dcl.type.elab#nt:elaborated-type-specifier),
in either case with a
[nested-name-specifier](https://eel.is/c++draft/expr.prim.id.qual#nt:nested-name-specifier)
ending with a
[simple-template-id](https://eel.is/c++draft/temp.names#nt:simple-template-id),
C, whose
[template-name](https://eel.is/c++draft/temp.names#nt:template-name)
names a class
template[.](https://eel.is/c++draft/temp.friend#5.sentence-2) The
template parameters of the template friend declaration shall be
deducible from C
([[temp.deduct.type]](https://eel.is/c++draft/temp.deduct.type))[.](https://eel.is/c++draft/temp.friend#5.sentence-3)
In this case, a member of a specialization S of the class template is a
friend of the class granting friendship if deduction of the template
parameters of C from S succeeds, and substituting the deduced template
arguments into the friend declaration produces a declaration that
corresponds to the member of the
specialization[.](https://eel.is/c++draft/temp.friend#5.sentence-4)

A useful example is provided after the paragraph:
https://eel.is/c++draft/temp.friend#example-4

.

Neither of Core issues is implemented, because we don't support
dependent nested friend specifiers.

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Co-authored-by: Shafik Yaghmour <shafik.yaghmour@intel.com>