Fix -fmodules build after rL323277

[arichardson]

Summary

Since rL323277 utils/TableGen/CodeGenSchedule.cpp pulls in "llvm/CodeGen/TargetOpcodes.h".
This currently attempts to build the whole llvm/CodeGen module which will try to add
LLVM/IR/Attributes.h. This headers #include "llvm/IR/Attributes.gen" which needs to be
built by tablegen so the build failed.

As a workaround I marked llvm/CodeGen/TargetOpcodes.h as a textual header. I am not sure if that is the correct solution but at least the build passes again for me.

[bkramer]

I went a slightly different route in rL323383, but this would've worked too. The layering between CodeGen and TableGen is messy, we'll have to clean that up eventually :(

include/llvm/module.modulemap

module CodeGen {
     exclude header "CodeGen/LinkAllAsmWriterComponents.h"
     exclude header "CodeGen/LinkAllCodegenComponents.h"
 
     // These are intended for (repeated) textual inclusion.
     textual header "CodeGen/CommandFlags.def"
     textual header "CodeGen/DIEValue.def"
     textual header "CodeGen/RuntimeLibcalls.def"
     textual header "CodeGen/TargetOpcodes.def"
+
+    // This is needed by tablegen which can't pull in all of the dependecies of
+    // llvm/CodeGen since some of those headers are generated by tablegen.
+    textual header "CodeGen/TargetOpcodes.h"
   }
 
   module Target {
     umbrella "Target"
     module * { export * }
   }
 }