Break module dependency llvm-tblgen -> LLVM_MC

[benlangmuir]

Summary

The intent seems to be that tblgen depends on LLVM_MC_TableGen rather than on LLVM_MC itself. Found this while investigating a build failure in llvm-tblgen that was ultimately fixed by 148071fbae3f9a101, but it's still good to fix the dependency.

llvm/include/llvm/module.modulemap

module LLVM_MC {
 
   umbrella "MC"
   module * { export * }
 }
 
 // Used by llvm-tblgen
 module LLVM_MC_TableGen {
   requires cplusplus
+  module MC_DecoderOps{ header "MC/MCDecoderOps.h" export * }
   module MC_LaneBitmask { header "MC/LaneBitmask.h" export * }
   module MC_InstrItineraries { header "MC/MCInstrItineraries.h" export * }
   module MC_Schedule { header "MC/MCSchedule.h" export * }
   module MC_SubtargetFeature { header "MC/SubtargetFeature.h" export * }
 }
 
 module LLVM_Object {
   requires cplusplus