[lld] [LLD][COFF] Fix handling of weak aliases referencing lazy symbols (PR #112243)

[Jacek Caban via llvm-commits]

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@@ -340,7 +340,10 @@ class Undefined : public Symbol {
   // If this symbol is external weak, try to resolve it to a defined
   // symbol by searching the chain of fallback symbols. Returns the symbol if
   // successful, otherwise returns null.
-  Defined *getWeakAlias();
+  Symbol *getWeakAlias();
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cjacek wrote:

The remaining callers use `getWeakAlias` to check if a symbol can be resolved. After the `run()` call, all such symbols should be replaced with a `Defined` symbol (assuming valid input, though a malformed archive index could disrupt this). This means that for many callers, `getWeakAlias` isn’t expected to handle lazy symbols, making it functionally equivalent to `getDefinedWeakAlias`. They only differ if new weak aliases are defined between `run()` calls.

In the test example from this PR, using `getDefinedWeakAlias` in `findMangle` would mostly work as well, but it's a bit more fragile. Processing `-export:func` would create a weak alias to `_func at 0`, but `-export:func2=func` would still consider it unresolvable and attempt to mangle it again. On the other hand, `getWeakAlias` returns the lazy symbol, so we know it will be resolvable and can skip the unnecessary mangling attempt.

https://github.com/llvm/llvm-project/pull/112243