[Sema] Always consider outer alternatives when resolving a decl ref.
Currently, when a type member has the same base name as a global declaration, one cannot refer to the global member from a method context, and the current diagnostics force the user to use a fully qualified name. This is a long-standing issue with the use of top-level `min` and `max` in an extension, and is causing unnecessarily verbosity in math algorithms in extensions for `ElementaryFunctions`-conforming types.
More context:
1. [Forum thread](https://forums.swift.org/t/elementaryfunctions-and-ambiguity-in-extensions/25297): `ElementaryFunctions` and ambiguity in extensions
2. [SR-456](https://bugs.swift.org/browse/SR-456): Confusing build error when calling 'max' function within 'extension Int'.
3. [SR-1772](https://bugs.swift.org/browse/SR-1772): File-level function with the same name as instance function not picked up by compiler.
This PR makes `UnresolvedDeclRefExpr` resolution always consider alternative candidates from the outer context. The following code will no longer trigger any error or warning.
```swift
import Foundation // which exports a top-level `sin(_:)`.
extension Float {
func foo() {
// Refers to top-level `sin(_:)`, not static method `Float.sin(_:)`.
_ = sin(self)
_ = sin(_:) as (Float) -> Float
}
}
extension Sequence {
func foo() {
// Refers to top-level `max(_:_:)`.
_ = max(1, 2)
}
}
```
swift
708bac74 - [Sema] Always consider outer alternatives when resolving a decl ref.
