Suppress Clang 21 -Wcharacter-conversion

The new Clang 21 warning is wreaking havoc in C++ projects that want
to use modern C++, because it not only warns about char8_t → char16_t,
but also about the completely harmless char16_t → char32_t conversions.

It has since been fixed, but suppress the warning globally until we
know which version has the fix (looks to be 21.1.4 at this point).

Upstream bug report: llvm/llvm-project#163719
More discussion in: llvm/llvm-project#138526
Upstream fix: llvm/llvm-project#163927

Pick-to: 6.8 6.5
Change-Id: I6409f6f6833131e3a77d81ab679125b2806e86a2
Reviewed-by: Edward Welbourne <[email protected]>
Reviewed-by: Ahmad Samir <[email protected]>
(cherry picked from commit f20f12b)
Reviewed-by: Qt Cherry-pick Bot <[email protected]>

Author: marcmutz

src/corelib/global/qcompilerdetection.h

@@ -1443,6 +1443,10 @@ QT_WARNING_DISABLE_MSVC(4706) /* assignment within conditional expression */
 QT_WARNING_DISABLE_MSVC(4355) /* 'this' : used in base member initializer list */
 QT_WARNING_DISABLE_MSVC(4710) /* function not inlined */
 QT_WARNING_DISABLE_MSVC(4530) /* C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc */
+#  elif defined(Q_CC_CLANG_ONLY)
+#    if Q_CC_CLANG >= 2100
+        QT_WARNING_DISABLE_CLANG("-Wcharacter-conversion") /* until https://github.com/llvm/llvm-project/issues/163719 is fixed */
+#    endif
 #  elif defined(Q_CC_BOR)
 #    pragma option -w-inl
 #    pragma option -w-aus