[clang] Improve error message for template at block scope

[chrchr-github]

int main() {
    template<typename T>
    struct S{};
}

clang:

<source>:2:5: error: expected expression
    2 |     template<typename T>
      |     ^

gcc:

<source>:2:5: error: a template declaration cannot appear at block scope
    2 |     template<typename T>
      |     ^~~~~~~~

msvc:

<source>(2): error C2951: template declarations are only permitted at global, namespace, or class scope
<source>(2): error C3857: 'S': multiple template parameter lists are not allowed

https://godbolt.org/z/5cGvn68xv

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (chrchr-github)

~~~c++ int main() { template<typename T> struct S{}; } ~~~ clang: ~~~ <source>:2:5: error: expected expression 2 | template<typename T> | ^ ~~~ gcc: ~~~ <source>:2:5: error: a template declaration cannot appear at block scope 2 | template<typename T> | ^~~~~~~~ ~~~ msvc: ~~~ <source>(2): error C2951: template declarations are only permitted at global, namespace, or class scope <source>(2): error C3857: 'S': multiple template parameter lists are not allowed ~~~ https://godbolt.org/z/5cGvn68xv

[shafik]

Yeah, that is not super helpful diagnostic.

[shafik]

duplicate: #17959