Add a virtual destructor to the AbiLayout base class.

[maleadt]

Since we're using ABI layout objects polymorphically (via std::unique_ptr<AbiLayout> abi), the base class needs a virtual destructor or the derived class' destructor will never trigger (it might even be UB).

This also squelches compile-time warnings:

In file included from src/codegen.cpp:1870:
In file included from src/intrinsics.cpp:7:
src/ccall.cpp:337:7: warning: 'AbiLayout' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
class AbiLayout {
      ^
In file included from src/codegen.cpp:1870:
In file included from src/intrinsics.cpp:7:
In file included from src/ccall.cpp:363:
src/abi_llvm.cpp:41:8: warning: 'ABI_LLVMLayout' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
struct ABI_LLVMLayout : AbiLayout {
       ^
In file included from src/codegen.cpp:1870:
In file included from src/intrinsics.cpp:7:
In file included from src/ccall.cpp:365:
src/abi_arm.cpp:24:8: warning: 'ABI_ARMLayout' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
struct ABI_ARMLayout : AbiLayout {
       ^
In file included from src/codegen.cpp:1870:
In file included from src/intrinsics.cpp:7:
In file included from src/ccall.cpp:366:
src/abi_aarch64.cpp:14:8: warning: 'ABI_AArch64Layout' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
struct ABI_AArch64Layout : AbiLayout {
       ^
In file included from src/codegen.cpp:1870:
In file included from src/intrinsics.cpp:7:
In file included from src/ccall.cpp:367:
src/abi_ppc64le.cpp:42:8: warning: 'ABI_PPC64leLayout' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
struct ABI_PPC64leLayout : AbiLayout {
       ^
In file included from src/codegen.cpp:1870:
In file included from src/intrinsics.cpp:7:
In file included from src/ccall.cpp:368:
src/abi_win32.cpp:40:8: warning: 'ABI_Win32Layout' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
struct ABI_Win32Layout : AbiLayout {
       ^
In file included from src/codegen.cpp:1870:
In file included from src/intrinsics.cpp:7:
In file included from src/ccall.cpp:369:
src/abi_win64.cpp:46:8: warning: 'ABI_Win64Layout' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
struct ABI_Win64Layout : AbiLayout {
       ^
In file included from src/codegen.cpp:1870:
In file included from src/intrinsics.cpp:7:
In file included from src/ccall.cpp:370:
src/abi_x86_64.cpp:41:8: warning: 'ABI_x86_64Layout' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
struct ABI_x86_64Layout : AbiLayout {
       ^
In file included from src/codegen.cpp:1870:
In file included from src/intrinsics.cpp:7:
In file included from src/ccall.cpp:371:
src/abi_x86.cpp:40:8: warning: 'ABI_x86Layout' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
struct ABI_x86Layout : AbiLayout {
       ^

[maleadt]

Fixed a similar warning in the second commit:

In file included from src/codegen.cpp:1870:
In file included from src/intrinsics.cpp:7:
src/ccall.cpp:769:7: warning: 'FunctionMover' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
class FunctionMover : public ValueMaterializer

FunctionMover is meant to be used non-polymorphically (ValueMaterializer's ctors are protected), so the base dtor isn't virtual. Making FunctionMover final and removing its virtual methods makes that we don't need a virtual dtor ourselves (only removing the virtual methods isn't sufficient because we inherit other virtual methods which we don't override).