Re-apply "[serialization] Reject loading a module with the wrong case."

This included a test that failed on case-sensitive filesystems. Test fixed.

(Aside: Why not just have this fail with "no such module"? Why use a different
error? Because even if "import FOO" picks up a module named 'Foo', there may
actually be a module named 'FOO' on the system (in another folder), and we
should be able to find that. Fixing that is tracked by rdar://problem/18691936.)

rdar://problem/15632996 (again)

Swift SVN r22856

include/swift/Serialization/ModuleFile.h

@@ -64,6 +64,9 @@ class ModuleFile : public LazyMemberLoader {
   llvm::BitstreamCursor SILCursor;
   llvm::BitstreamCursor SILIndexCursor;
 
+  /// The name of the module.
+  StringRef Name;
+
   /// The data blob containing all of the module's identifiers.
   StringRef IdentifierData;
 
@@ -478,6 +481,11 @@ public:
     return ModuleInputBuffer->getBufferIdentifier();
   }
 
+  /// Returns the module name as stored in the serialized data.
+  StringRef getModuleName() const {
+    return Name;
+  }
+
   /// AST-verify imported decls.
   ///
   /// Has no effect in NDEBUG builds.