Add option to disable module loading.

This patch was created by Lawrence Crowl and reviewed in:
http://llvm-reviews.chandlerc.com/D963

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187738 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/Modules.rst b/docs/Modules.rst
index cd430a2..6ac334d 100644
--- a/docs/Modules.rst
+++ b/docs/Modules.rst
@@ -148,6 +148,8 @@
 .. note::
 
   To actually see any benefits from modules, one first has to introduce module maps for the underlying C standard library and the libraries and headers on which it depends. The section `Modularizing a Platform`_ describes the steps one must take to write these module maps.
+  
+One can use module maps without modules to check the integrity of the use of header files. To do this, use the ``-fmodule-maps`` option instead of the ``-fmodules`` option.
 
 Compilation model
 -----------------
@@ -165,6 +167,9 @@
 ``-fcxx-modules``
   Enable the modules feature for C++ (EXPERIMENTAL and VERY BROKEN).
 
+``-fmodule-maps``
+  Enable interpretation of module maps (EXPERIMENTAL). This option is implied by ``-fmodules``.
+
 ``-fmodules-cache-path=<directory>``
   Specify the path to the modules cache. If not provided, Clang will select a system-appropriate default.