[C++][Module] Improve diagnostic 'export declaration can only be used within a module purview'

[yronglin]

module M; // #M

export int b2; // expected-error {{export declaration can only be used within a module purview}}

As Michael's suggestion in #148861 (comment), This should say "within the purview of a module interface" for this case, or just "within a module interface" to be less pedantic.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: None (yronglin)

```cpp module M; // #M

export int b2; // expected-error {{export declaration can only be used within a module purview}}

As Michael's suggestion in https://github.com/llvm/llvm-project/pull/148861#discussion_r2208847734, This should say "within the purview of a module interface" for this case, or just "within a module interface" to be less pedantic.
</details>

[llvmbot]

@llvm/issue-subscribers-c-20

Author: None (yronglin)

```cpp module M; // #M

export int b2; // expected-error {{export declaration can only be used within a module purview}}

As Michael's suggestion in https://github.com/llvm/llvm-project/pull/148861#discussion_r2208847734, This should say "within the purview of a module interface" for this case, or just "within a module interface" to be less pedantic.
</details>

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor is working on this issue. If someone is assigned to the issue or claimed to be working on it, ping the person. After one week without a response, the assignee may be changed.
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3. Fix the issue locally.
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If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: None (yronglin)

```cpp module M; // #M

export int b2; // expected-error {{export declaration can only be used within a module purview}}

As Michael's suggestion in https://github.com/llvm/llvm-project/pull/148861#discussion_r2208847734, This should say "within the purview of a module interface" for this case, or just "within a module interface" to be less pedantic.
</details>