[BuildSystem] Add diagnosticFiles to a module build description

This would make it possible to access diagnostic files through a
generic build system interface.

Author: xedin

Sources/Build/BuildDescription/ModuleBuildDescription.swift

@@ -140,6 +140,14 @@ public enum ModuleBuildDescription: SPMBuildCore.ModuleBuildDescription {
         }
     }
 
+    public var diagnosticFiles: [AbsolutePath] {
+        switch self {
+        case .swift(let buildDescription):
+            buildDescription.diagnosticFiles
+        case .clang(_):
+            []
+        }
+    }
     /// Determines the arguments needed to run `swift-symbolgraph-extract` for
     /// this module.
     public func symbolGraphExtractArguments() throws -> [String] {

Sources/SPMBuildCore/BuildSystem/BuildSystem.swift

@@ -92,6 +92,10 @@ public protocol ModuleBuildDescription {
     /// The build parameters.
     var buildParameters: BuildParameters { get }
 
+    /// The diagnostic file locations for all the source files
+    /// associated with this module.
+    var diagnosticFiles: [AbsolutePath] { get }
+
     /// FIXME: This shouldn't be necessary and ideally
     /// there should be a way to ask build system to
     /// introduce these arguments while building for symbol