don't drop underscored protocol implementation symbols (#75848)

rdar://133086270

test/SymbolGraph/Symbols/SkipProtocolImplementations.swift

@@ -25,15 +25,21 @@
 // Same for ExtraStruct.Inner
 // CHECK-DAG: s:27SkipProtocolImplementations11ExtraStructV5InnerV
 
+// HiddenStruct.T should be present because its source protocol is underscored
+// CHECK-DAG: "precise": "s:27SkipProtocolImplementations12HiddenStructV1TV",
+
 // CHECK-LABEL: "relationships": [
 
 // we want to make sure that the conformance relationship itself stays
 // CHECK-DAG: conformsTo
 
-// SomeStruct.otherFunc() and ExtraStruct.Inner should be the only ones with sourceOrigin information
+// There should only be three symbols with sourceOrigin information:
+// - SomeStruct.otherFunc()
+// - ExtraStruct.Inner
+// - HiddenStruct.T
 // (ExtraStruct.Inner will have two sourceOrigins because it has two relationships: a memberOf and a
 // conformsTo)
-// COUNT-COUNT-3: sourceOrigin
+// COUNT-COUNT-4: sourceOrigin
 // COUNT-NOT: sourceOrigin
 
 public protocol SomeProtocol {
@@ -73,3 +79,11 @@ public struct ExtraStruct: OtherProtocol {
 }
 
 extension ExtraStruct.Inner: Sendable {}
+
+public protocol _HiddenProtocol {
+    associatedtype T
+}
+
+public struct HiddenStruct: _HiddenProtocol {
+    public struct T {}
+}