Index: llvm/trunk/CMakeLists.txt
===================================================================
--- llvm/trunk/CMakeLists.txt
+++ llvm/trunk/CMakeLists.txt
@@ -187,6 +187,11 @@
 
 option(LLVM_INSTALL_TOOLCHAIN_ONLY "Only include toolchain files in the 'install' target." OFF)
 
+# Unfortunatly Clang is too eager to search directories for module maps, which can cause the
+# installed version of the maps to be found when building LLVM from source. Therefore we turn off
+# the installation by default. See llvm.org/PR31905.
+option(LLVM_INSTALL_MODULEMAPS "Install the modulemap files in the 'install' target." OFF)
+
 option(LLVM_USE_FOLDERS "Enable solution folders in Visual Studio. Disable for Express versions." ON)
 if ( LLVM_USE_FOLDERS )
   set_property(GLOBAL PROPERTY USE_FOLDERS ON)
@@ -973,6 +978,20 @@
     PATTERN ".svn" EXCLUDE
     )
 
+  if (LLVM_INSTALL_MODULEMAPS)
+    install(DIRECTORY include/llvm include/llvm-c
+            DESTINATION include
+            COMPONENT llvm-headers
+            FILES_MATCHING
+            PATTERN "module.modulemap"
+            )
+    install(FILES include/llvm/module.install.modulemap
+            DESTINATION include/llvm
+            COMPONENT llvm-headers
+            RENAME "module.extern.modulemap"
+            )
+  endif(LLVM_INSTALL_MODULEMAPS)
+
   # Installing the headers needs to depend on generating any public
   # tablegen'd headers.
   add_custom_target(llvm-headers DEPENDS intrinsics_gen)
Index: llvm/trunk/include/llvm/module.extern.modulemap
===================================================================
--- llvm/trunk/include/llvm/module.extern.modulemap
+++ llvm/trunk/include/llvm/module.extern.modulemap
@@ -0,0 +1,5 @@
+module LLVM_Extern_Config_Def {}
+module LLVM_Extern_IR_Attributes_Gen {}
+module LLVM_Extern_IR_Intrinsics_Gen {}
+module LLVM_Extern_IR_Intrinsics_Enum {}
+module LLVM_Extern_Utils_DataTypes {}
Index: llvm/trunk/include/llvm/module.install.modulemap
===================================================================
--- llvm/trunk/include/llvm/module.install.modulemap
+++ llvm/trunk/include/llvm/module.install.modulemap
@@ -0,0 +1,27 @@
+
+module LLVM_Extern_Config_Def {
+ textual header "Config/AsmParsers.def"
+ textual header "Config/AsmPrinters.def"
+ textual header "Config/Disassemblers.def"
+ textual header "Config/Targets.def"
+ export *
+}
+
+module LLVM_Extern_IR_Attributes_Gen {
+  textual header "IR/Attributes.gen"
+  textual header "IR/Attributes.inc"
+}
+
+module LLVM_Extern_IR_Intrinsics_Gen {
+  textual header "IR/Intrinsics.gen"
+  textual header "IR/Intrinsics.inc"
+}
+
+module LLVM_Extern_IR_Intrinsics_Enum {
+  textual header "IR/IntrinsicEnums.inc"
+}
+
+module LLVM_Extern_Utils_DataTypes {
+  header "Support/DataTypes.h"
+  export *
+}
Index: llvm/trunk/include/llvm/module.modulemap
===================================================================
--- llvm/trunk/include/llvm/module.modulemap
+++ llvm/trunk/include/llvm/module.modulemap
@@ -36,6 +36,7 @@
 
 module LLVM_Bitcode { requires cplusplus umbrella "Bitcode" module * { export * } }
 
+
 module LLVM_BinaryFormat {
     requires cplusplus
     umbrella "BinaryFormat" module * { export * }
@@ -63,7 +64,12 @@
     textual header "BinaryFormat/MsgPack.def"
 }
 
-module LLVM_Config { requires cplusplus umbrella "Config" module * { export * } }
+module LLVM_Config {
+  requires cplusplus
+  umbrella "Config"
+  extern module LLVM_Extern_Config_Def "module.extern.modulemap"
+  module * { export * }
+}
 
 module LLVM_DebugInfo {
   requires cplusplus
@@ -181,7 +187,11 @@
 
   // Attributes.h
   module IR_Argument { header "IR/Argument.h" export * }
-  module IR_Attributes { header "IR/Attributes.h" export * }
+  module IR_Attributes {
+    header "IR/Attributes.h"
+    extern module LLVM_Extern_IR_Attributes_Gen "module.extern.modulemap"
+    export *
+  }
   module IR_CallSite { header "IR/CallSite.h" export * }
   module IR_ConstantFolder { header "IR/ConstantFolder.h" export * }
   module IR_GlobalVariable { header "IR/GlobalVariable.h" export * }
@@ -207,7 +217,12 @@
   module IR_Verifier { header "IR/Verifier.h" export * }
   module IR_InstIterator { header "IR/InstIterator.h" export * }
   module IR_InstVisitor { header "IR/InstVisitor.h" export * }
-  module IR_Intrinsics { header "IR/Intrinsics.h" export * }
+  module IR_Intrinsics {
+    header "IR/Intrinsics.h"
+    extern module LLVM_Extern_IR_Intricsics_Gen "module.extern.modulemap"
+    extern module LLVM_Extern_IR_Intrinsics_Enum "module.extern.modulemap"
+    export *
+  }
   module IR_IntrinsicInst { header "IR/IntrinsicInst.h" export * }
   module IR_PatternMatch { header "IR/PatternMatch.h" export * }
   module IR_Statepoint { header "IR/Statepoint.h" export * }
@@ -284,6 +299,8 @@
   module * { export * }
 }
 
+extern module LLVM_Extern_Utils_DataTypes "module.extern.modulemap"
+
 // A module covering ADT/ and Support/. These are intertwined and
 // codependent, and notionally form a single module.
 module LLVM_Utils {