[IRInterpreter] Return zero address for missing weak function (#93548)

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
#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;