[Modules] Add module for Config/llvm-config.h

Besides all the goodness from modularizing a header, this is necessary
to compile ToT with modules with the clang host compiler from Xcode 9 in
macOS 10.13, which our bots don't use yet.

rdar://problem/35038151

llvm-svn: 316414

include/llvm/module.modulemap.build

@@ -7,3 +7,7 @@ module LLVM_Config_ABI_Breaking {
   header "Config/abi-breaking.h"
   export *
 }
+module LLVM_Config_Config {
+  header "Config/llvm-config.h"
+  export *
+}