[ELF] Make shouldAddProvideSym return values consistent when demoted to Undefined

Case: `PROVIDE(f1 = bar);` when both `f1` and `bar` are in separate
sections that would be discarded by GC.

Due to `demoteDefined`, `shouldAddProvideSym(f1)` may initially return
false (when Defined) and then return true (been demoted to Undefined).

```
addScriptReferencedSymbolsToSymTable
  shouldAddProvideSym(f1): false
  // the RHS (bar) is not added to `referencedSymbols` and may be GCed
declareSymbols
  shouldAddProvideSym(f1): false
markLive
demoteSymbolsAndComputeIsPreemptible
  // demoted f1 to Undefined
processSymbolAssignments
  addSymbol
    shouldAddProvideSym(f1): true
```

The inconsistency can cause `cmd->expression()` in `addSymbol` to be
evaluated, leading to `symbol not found: bar` errors (since `bar` in the
RHS is not in `referencedSymbols` and is GCed) (#111478).

Fix this by adding a `sym->isUsedInRegularObj` condition, making
`shouldAddProvideSym(f1)` values consistent. In addition, we need a
`sym->exportDynamic` condition to keep provide-shared.s working.

Fixes: ebb326a51fec37b5a47e5702e8ea157cd4f835cd

Pull Request: https://github.com/llvm/llvm-project/pull/111945

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