[clang][Sema] Fix diagnostic for function overloading in extern "C" (#106033)

Fixes #80235

When trying to overload a function within `extern "C"`, the diagnostic
`functions that differ only in their return type cannot be overloaded`
is given. This diagnostic is inappropriate because overloading is
basically not allowed in the C language. However, if the redeclared
function has the `((overloadable))` attribute, it should be diagnosed as
`functions that differ only in their return type cannot be overloaded`.

This patch uses `isExternC()` to provide an appropriate diagnostic
during the diagnostic process. `isExternC()` updates the linkage
information cache internally, so calling it before merging functions can
cause clang to crash. An example is declaring `static void foo()` and
`void foo()` within an `extern "C"` block. Therefore, I decided to call
`isExternC()` after the compilation error is confirmed and select the
diagnostic message. The diagnostic message is `conflicting types for
'func'` similar to the diagnostic in C, and `functions that differ only
in their return type cannot be overloaded` if the `((overloadable))`
attribute is given.

Regression tests verify that the expected diagnostics are given when
trying to overload functions within `extern "C"` and when the
`((overloadable))` attribute is present.

---------

Co-authored-by: Sirraide <aeternalmail@gmail.com>

0 files changed, 0 insertions, 0 deletions