[Xcodeproj] Add search path to ClangModule's own include dir

- SR-2526
- <rdar://problem/28074850> SR-2526 [xcodeproj] Add "include" dir to C
  target's header search paths

Author: aciidgh

Sources/Xcodeproj/Module+PBXProj.swift

@@ -157,7 +157,7 @@ extension Module  {
 
     var headerSearchPaths: (key: String, value: String)? {
         let headerPathKey = "HEADER_SEARCH_PATHS"
-        let headerPaths = dependencies.flatMap { module -> AbsolutePath? in
+        var headerPaths = dependencies.flatMap { module -> AbsolutePath? in
             switch module {
             case let cModule as CModule:
                 return cModule.path
@@ -168,6 +168,11 @@ extension Module  {
             }
         }
 
+        // For ClangModules add implicit search path to its own include directory.
+        if case let clangModule as ClangModule = self {
+            headerPaths.append(clangModule.includeDir)
+        }
+
         guard !headerPaths.isEmpty else { return nil }
 
         if headerPaths.count == 1, let first = headerPaths.first {