Module use declarations (II)

Review: http://llvm-reviews.chandlerc.com/D1546.

I have picked up this patch form Lawrence
(http://llvm-reviews.chandlerc.com/D1063) and did a few changes.

From the original change description (updated as appropriate):
This patch adds a check that ensures that modules only use modules they
have so declared. To this end, it adds a statement on intended module
use to the module.map grammar:

  use module-id

A module can then only use headers from other modules if it 'uses' them.
This enforcement is off by default, but may be turned on with the new
option -fmodules-decluse.

When enforcing the module semantics, we also need to consider a source
file part of a module. This is achieved with a compiler option

-fmodule-name=<module-id>.

The compiler at present only applies restrictions to the module directly
being built.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191283 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/Tools.cpp b/lib/Driver/Tools.cpp
index 23f3648..6a803cc 100644
--- a/lib/Driver/Tools.cpp
+++ b/lib/Driver/Tools.cpp
@@ -3034,6 +3034,13 @@
     CmdArgs.push_back("-fmodule-maps");
   }
 
+  // -fmodule-decluse checks that modules used are declared so (off by default).
+  if (Args.hasFlag(options::OPT_fmodules_decluse,
+                   options::OPT_fno_modules_decluse,
+                   false)) {
+    CmdArgs.push_back("-fmodule-decluse");
+  }
+
   // If a module path was provided, pass it along. Otherwise, use a temporary
   // directory.
   if (Arg *A = Args.getLastArg(options::OPT_fmodules_cache_path)) {