tst_QByteArrayView: expose constExpr() test to non-ubsan GCC builds

Because of https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71962, and
because marking constExpr() as __attribute__((no_sanitize_undefined))
doesn't fix a thing, we opted to disable the triggering parts of
constExpr() from all GCC builds.

This is, of course, unfortunate, since it meaningfully reduces
compile-time coverage in the general case for a rather obscure build
type most won't ever use.

While GCC doesn't give us a predefined macro to check for in the .cpp
file, the cmake build system knows whether we use ubsan, so just
define a macro of our own.

As a drive-by, simplify GCC detection by using Q_CC_GCC_ONLY.

Amends de6a004bc5.

Pick-to: 6.7 6.5 6.2
Task-number: QTBUG-101307
Change-Id: I4be5bd103b9d2386b2ac9fd22e0c34f9c63fee04
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/corelib/text/qbytearrayview/CMakeLists.txt

@@ -15,3 +15,10 @@ qt_internal_add_test(tst_qbytearrayview
     SOURCES
         tst_qbytearrayview.cpp
 )
+
+if(QT_FEATURE_sanitize_undefined)
+    qt_internal_extend_target(tst_qbytearrayview
+        DEFINES
+            QT_SANITIZE_UNDEFINED # GCC (in)famously doesn't provide a predefined macro for this
+    )
+endif()

tests/auto/corelib/text/qbytearrayview/tst_qbytearrayview.cpp

@@ -262,10 +262,9 @@ void tst_QByteArrayView::constExpr() const
         static_assert(!bv2.empty());
         static_assert(bv2.size() == 5);
     }
-#if !defined(Q_CC_GNU) || defined(Q_CC_CLANG)
+#if !defined(Q_CC_GNU_ONLY) || !defined(QT_SANITIZE_UNDEFINED)
     // Below checks are disabled because of a compilation issue with GCC and
     // -fsanitize=undefined. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71962.
-    // Note: Q_CC_GNU is also defined for Clang, so we need to check that too.
     {
         static constexpr char hello[] = "Hello";
         constexpr QByteArrayView bv(hello);