Module: add -fprebuilt-module-path to support loading prebuilt modules.

In this mode, there is no need to load any module map and the programmer can
simply use "@import" syntax to load the module directly from a prebuilt
module path. When loading from prebuilt module path, we don't support
rebuilding of the module files and we ignore compatible configuration
mismatches.

rdar://27290316
Differential Revision: http://reviews.llvm.org/D23125

llvm-svn: 279096
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index aafbb48..3a34f85 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -2805,6 +2805,7 @@
     switch (M.Kind) {
     case serialization::MK_ImplicitModule:
     case serialization::MK_ExplicitModule:
+    case serialization::MK_PrebuiltModule:
       return true; // skip dependencies.
     case serialization::MK_PCH:
       Mod = &M;