[IRInterpreter] Return zero address for missing weak function

If a weak function is missing, still return it's address (zero)
rather than failing interpretation. Otherwise we have a mismatch
between Interpret() and CanInterpret() resulting in failures that
would not occur with JIT execution.

Alternatively, we could try to look for weak symbols in
CanInterpret() and generally reject them there.

This is the root cause for the issue exposed by
llvm#92885. Previously,
the case affected by that always fell back to JIT because an
icmp constant expression was used, which is not supported by the
interpreter. Now a normal icmp instruction is used, which is
supported. However, we fail to interpret due to incorrect
handling of weak function addresses.

Author: nikic

lldb/source/Expression/IRInterpreter.cpp

@@ -264,7 +264,7 @@ class InterpreterStackFrame {
         lldb_private::ConstString name(constant_func->getName());
         bool missing_weak = false;
         lldb::addr_t addr = m_execution_unit.FindSymbol(name, missing_weak);
-        if (addr == LLDB_INVALID_ADDRESS || missing_weak)
+        if (addr == LLDB_INVALID_ADDRESS)
           return false;
         value = APInt(m_target_data.getPointerSizeInBits(), addr);
         return true;