[OpenMP] Fixed the issue that CMake variables for OpenMP were not passed through when building OpenMP with LLVM_ENABLE_RUNTIMES

[tianshilei1992]

Summary

Currently when building OpenMP along with LLVM, CMake variables for OpenMP (prefix with LIBOMP and LIBOMPTARGET) will not be passed through because by default it uses the prefix of the runtime name, aka OPENMP in this case. This patch fixed this issue.

[jdoerfert]

LGTM, cool! Thanks for tracking this down!

[jdoerfert]

I'd remove the [NFC] tag though.

llvm/runtimes/CMakeLists.txt

${error} Set RUNTIMES_BUILD_ALLOW_DARWIN to allow a single darwin triple.")
   # We create a list the names of all the runtime projects in all uppercase and
   # with dashes turned to underscores. This gives us the CMake variable prefixes
   # for all variables that will apply to runtimes.
   foreach(entry ${runtimes})
     get_filename_component(projName ${entry} NAME)
     string(REPLACE "-" "_" canon_name ${projName})
     string(TOUPPER ${canon_name} canon_name)
     list(APPEND prefixes ${canon_name})
+    if (${canon_name} STREQUAL "OPENMP")
+      list(APPEND prefixes "LIBOMP" "LIBOMPTARGET")
+    endif()
     # Many compiler-rt options start with SANITIZER_ rather than COMPILER_RT_,
     # so when compiler-rt is enabled, consider both.
     if(canon_name STREQUAL "COMPILER_RT")
       list(APPEND prefixes SANITIZER)
     endif()
 
     string(FIND ${projName} "lib" LIB_IDX)
     if(LIB_IDX EQUAL 0)