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/docs/Modules.rst b/clang/docs/Modules.rst
index 0ea3b5b..4187654 100644
--- a/clang/docs/Modules.rst
+++ b/clang/docs/Modules.rst
@@ -213,6 +213,9 @@
 ``-fmodule-file=<file>``
   Load the given precompiled module file.
 
+``-fprebuilt-module-path=<directory>``
+  Specify the path to the prebuilt modules. If specified, we will look for modules in this directory for a given top-level module name. We don't need a module map for loading prebuilt modules in this directory and the compiler will not try to rebuild these modules. This can be specified multiple times.
+
 Module Semantics
 ================