Lower the "ambiguous display name" diagnostic to a warning for some names.

This PR modifies the behaviour of this compile-time macro diagnostic:

> 🛑 "Attribute 'Test' specifies display name 'foo' for function with implicit
> display name 'bar'

If `bar` (in the above context) is _not_ considered a raw identifier by the
language, we emit a warning instead of an error. This will allow us to adjust
display-name-from-backticked-name inference (see #1174) without introducing a
source-breaking change for a declaration such as:

```swift
@test("subscript([K]) operator")
func `subscript`()
```

(The above is a real-world test function in our own package that would be
impacted.)

Note that we don't actually have a code path that triggers this warning yet.
 #1174, if approved and merged, would introduce such a code path.

Author: grynspan

Sources/TestingMacros/Support/AttributeDiscovery.swift

@@ -144,8 +144,9 @@ struct AttributeInfo {
        let rawIdentifier = namedDecl.name.rawIdentifier {
       if let displayName, let displayNameArgument {
         context.diagnose(.declaration(namedDecl, hasExtraneousDisplayName: displayName, fromArgument: displayNameArgument, using: attribute))
+      } else {
+        displayName = StringLiteralExprSyntax(content: rawIdentifier)
       }
-      displayName = StringLiteralExprSyntax(content: rawIdentifier)
     }
 
     // Remove leading "Self." expressions from the arguments of the attribute.

Sources/TestingMacros/Support/DiagnosticMessage.swift

@@ -657,10 +657,16 @@ struct DiagnosticMessage: SwiftDiagnostics.DiagnosticMessage {
     fromArgument argumentContainingDisplayName: LabeledExprListSyntax.Element,
     using attribute: AttributeSyntax
   ) -> Self {
-    Self(
+    // If the name of the ambiguously-named symbol should be derived from a raw
+    // identifier, this situation is an error. If the name is not raw but is
+    // still surrounded by backticks (e.g. "func `foo`()" or "struct `if`") then
+    // lower the severity to a warning. That way, existing code structured this
+    // way doesn't suddenly fail to build.
+    let severity: DiagnosticSeverity = (decl.name.rawIdentifier != nil) ? .error : .warning
+    return Self(
       syntax: Syntax(decl),
-      message: "Attribute \(_macroName(attribute)) specifies display name '\(displayNameFromAttribute.representedLiteralValue!)' for \(_kindString(for: decl)) with implicit display name '\(decl.name.rawIdentifier!)'",
-      severity: .error,
+      message: "Attribute \(_macroName(attribute)) specifies display name '\(displayNameFromAttribute.representedLiteralValue!)' for \(_kindString(for: decl)) with implicit display name '\(decl.name.textWithoutBackticks)'",
+      severity: severity,
       fixIts: [
         FixIt(
           message: MacroExpansionFixItMessage("Remove '\(displayNameFromAttribute.representedLiteralValue!)'"),