This fixes an UB cast when we call this on a ClassFunctionDeclaration
with setDeclarationIsDefinition(true) as shown in this UBSAN report:

```
/home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/language/duchain/functiondefinition.cpp:93:49: runtime error: downcast of address 0x6060005c72c0 which does not point to an object of type 'FunctionDefinition'
0x6060005c72c0: note: object is of type 'KDevelop::ClassFunctionDeclaration'
 26 02 00 76  f0 06 b1 d3 05 7f 00 00  a0 64 21 00 b0 60 00 00  50 6e 22 00 20 60 00 00  10 2b 01 01
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'KDevelop::ClassFunctionDeclaration'
    #0 0x7f05d2c5377f in KDevelop::FunctionDefinition::definition(KDevelop::Declaration const*) /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/language/duchain/functiondefinition.cpp:93
    #1 0x7f05dddae858 in accept /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/plugins/clang/duchain/documentfinderhelpers.cpp:63
    #2 0x7f05d2f7e0c2 in KDevelop::DUChainUtils::collectItems(KDevelop::DUContext*, KDevelop::DUChainUtils::DUChainItemFilter&) /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/language/duchain/duchainutils.cpp:422
    #3 0x7f05d2f7e4ed in KDevelop::DUChainUtils::collectItems(KDevelop::DUContext*, KDevelop::DUChainUtils::DUChainItemFilter&) /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/language/duchain/duchainutils.cpp:432
    #4 0x7f05dddacb36 in duchainBuddyFile /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/plugins/clang/duchain/documentfinderhelpers.cpp:136
    #5 0x7f05dddb2fcc in DocumentFinderHelpers::potentialBuddies(QUrl const&, bool) /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/plugins/clang/duchain/documentfinderhelpers.cpp:256
    #6 0x7f05dddb3a5d in DocumentFinderHelpers::sourceForHeader(QString const&) /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/plugins/clang/duchain/documentfinderhelpers.cpp:272
    #7 0x7f05dda0a7d5 in ClangRefactoring::moveIntoSource(KDevelop::IndexedDeclaration const&) /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/plugins/clang/codegen/clangrefactoring.cpp:158
    #8 0x563aae0570b3 in TestAssistants::testMoveIntoSource() /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/plugins/clang/tests/test_assistants.cpp:686
    #9 0x563aae01da1f in TestAssistants::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) plugins/clang/tests/test_assistants_autogen/EWIEGA46WW/moc_test_assistants.cpp:127
    #10 0x7f05c7f8458a in QMetaMethod::invoke(QObject*, Qt::ConnectionType, QGenericReturnArgument, QGenericArgument, QGenericArgument, QGenericArgument, QGenericArgument, QGenericArgument, QGenericArgument, QGenericArgument, QGenericArgument, QGenericArgument, QGenericArgument) const (/usr/lib/libQt5Core.so.5+0x2c458a)
    #11 0x7f05df029e96  (/usr/lib/libQt5Test.so.5+0x18e96)
    #12 0x7f05df02a750  (/usr/lib/libQt5Test.so.5+0x19750)
    #13 0x7f05df02ad03  (/usr/lib/libQt5Test.so.5+0x19d03)
    #14 0x7f05df02b1cd in QTest::qRun() (/usr/lib/libQt5Test.so.5+0x1a1cd)
    #15 0x7f05df02b57d in QTest::qExec(QObject*, int, char**) (/usr/lib/libQt5Test.so.5+0x1a57d)
    #16 0x563aae03e824 in main /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/plugins/clang/tests/test_assistants.cpp:61
    #17 0x7f05c6e72151 in __libc_start_main (/usr/lib/libc.so.6+0x28151)
    #18 0x563aae01d1bd in _start (/home/milian/projects/kf5/build-dbg/extragear/kdevelop/kdevelop-sanitized/bin/test_assistants+0xe41bd)
```