Removed CMake cache upgrade code from 2011

[gribozavr]

Summary

This code was added in r141266 to make a breaking change to CMake, but
still be compatible with existing cache files. The cache files from
2011 are irrelevant today in 2019.

[smeenai]

LGTM, thanks!

llvm/cmake/modules/TableGen.cmake

macro(add_tablegen target project)
   endif()
 
   add_llvm_executable(${target} DISABLE_LLVM_LINK_LLVM_DYLIB ${ARGN})
   set(LLVM_LINK_COMPONENTS ${${target}_OLD_LLVM_LINK_COMPONENTS})
 
   set(${project}_TABLEGEN "${target}" CACHE
       STRING "Native TableGen executable. Saves building one when cross-compiling.")
 
-  # Upgrade existing LLVM_TABLEGEN setting.
-  if(${project} STREQUAL LLVM)
-    if(${LLVM_TABLEGEN} STREQUAL tblgen)
-      set(LLVM_TABLEGEN "${target}" CACHE
-          STRING "Native TableGen executable. Saves building one when cross-compiling."
-          FORCE)
-    endif()
-  endif()
-
   # Effective tblgen executable to be used:
   set(${project}_TABLEGEN_EXE ${${project}_TABLEGEN} PARENT_SCOPE)
   set(${project}_TABLEGEN_TARGET ${${project}_TABLEGEN} PARENT_SCOPE)
 
   if(LLVM_USE_HOST_TOOLS)
     if( ${${project}_TABLEGEN} STREQUAL "${target}" )
       build_native_tool(${target} ${project}_TABLEGEN_EXE DEPENDS ${target})
       set(${project}_TABLEGEN_EXE ${${project}_TABLEGEN_EXE} PARENT_SCOPE)