[SYCL] Emit vtables of classes with indirectly_callable virtual functions in every translation unit that references the vtable

[lbushi25]

SYCLVirtualFunctionsAnalysisPass propagates the "calls-indirectly" attribute onto kernels by walking from a virtual function carrying "indirectly-callable" to the vtables which reference it, and from there to the kernels which reference those vtables. That only works if both the vtable initializer and the attribute are visible in the module being analyzed, which was not the case when a virtual function was defined out-of-line:

• "indirectly-callable" was only attached to function definitions, so a translation unit which only saw a declaration lost the information;
• ModuleBuilder::HandleVTable bailed out unconditionally for SYCL device compilation, so a vtable whose key function is defined in another translation unit was never emitted at all and the analysis had nothing to look at.

As a result a kernel which merely constructed an object of a polymorphic class was not marked as using virtual functions, and runtime linking later failed with undefined references to the virtual functions from the vtable.

This does the following, all restricted to SYCL device compilation and to classes which declare (or inherit) an 'indirectly_callable' virtual function:

• attach compile-time properties to function declarations as well, not just to definitions;
• force the vtable to be emitted in every translation unit which references the vtable, regardless of where the key function is defined;
• give such vtables linkonce_odr linkage so that emitting them more than once is not a problem once all device code is linked together, and so that they can be duplicated across device images produced by device code split - which is what VTables are propagated into every device image we produce during the split, leading to multiple definitions error #15069 was about.

Vtables of polymorphic classes unrelated to the virtual functions extension keep the previous behaviour of not being defined in device code at all.

The three previously XFAILed end-to-end tests are un-XFAILed and gain -O0 RUN lines, since vtable linkage depends on the optimization level.

Closes: #15071
Closes: #15069

[YuriPlyakhin]

llvm/test/SYCLLowerIR/SYCLVirtualFunctionsAnalysis/calls-indirectly-propagation-5.ll LGTM