[CMake] add_tablegen: Rename host tablegen to `${target}-host"

[chapuni]

Summary

Since llvm-min-tblgen has been introduced, llvm-min-tblgen-host
would make more sense than LLVM_HEADERS-tablegen-host.

Depends on D146352

[mstorsjo]

LGTM, although the commit message is a bit cryptic. What about:

This is a pure refactoring and simplification, allowing decoupling ${target} from ${project} in the future.

llvm/cmake/modules/TableGen.cmake

macro(add_tablegen target project)
   if(LLVM_USE_HOST_TOOLS)
     if( ${${project}_TABLEGEN} STREQUAL "${target}" )
       # The NATIVE tablegen executable *must* depend on the current target one
       # otherwise the native one won't get rebuilt when the tablgen sources
       # change, and we end up with incorrect builds.
       build_native_tool(${target} ${project}_TABLEGEN_EXE DEPENDS ${target})
       set(${project}_TABLEGEN_EXE ${${project}_TABLEGEN_EXE} PARENT_SCOPE)
 
-      add_custom_target(${project}-tablegen-host DEPENDS ${${project}_TABLEGEN_EXE})
-      set(${project}_TABLEGEN_TARGET ${project}-tablegen-host PARENT_SCOPE)
+      add_custom_target(${target}-host DEPENDS ${${project}_TABLEGEN_EXE})
+      set(${project}_TABLEGEN_TARGET ${target}-host PARENT_SCOPE)
 
       # If we're using the host tablegen, and utils were not requested, we have no
       # need to build this tablegen.
       if ( NOT LLVM_BUILD_UTILS )
         set_target_properties(${target} PROPERTIES EXCLUDE_FROM_ALL ON)
       endif()
     endif()
   endif()