[OpenMP] Fix import library installation with MinGW

[mati865]

Summary

Somehow I missed when submitting https://reviews.llvm.org/D85210 to review even though it was present in original patch for MSYS2 package.
Without this change CMake tries to install libomp.a (which doesn't exist for shared build) instead of libomp.dll.a.

Should not affect MSVC at all: https://gitlab.kitware.com/cmake/cmake/-/blob/5e212cafba0f2266510841eea3f5c7b79fde9449/Modules/Platform/Windows.cmake#L16 but makes big difference

[AndreyChurbanov]

LGTM

[mati865]

I don't have write permission, could somebody commit it for me?
Thanks in advance.

[hans]

Cherry-picked to 11.x as 522d80ab553b42e2feadfd4178932069dfc51d3f.

openmp/runtime/src/CMakeLists.txt

if(WIN32)
   libomp_list_to_string("${LIBOMP_MASM_DEFINITIONS}" LIBOMP_MASM_DEFINITIONS)
   set_property(SOURCE z_Windows_NT-586_asm.asm APPEND_STRING PROPERTY COMPILE_FLAGS " ${LIBOMP_MASM_DEFINITIONS}")
   set_source_files_properties(thirdparty/ittnotify/ittnotify_static.cpp PROPERTIES COMPILE_DEFINITIONS "UNICODE")
 
   # Create Windows import library
   # the import library is "re-linked" to include kmp_import.cpp which prevents
   # linking of both Visual Studio OpenMP and newly built OpenMP
   set_source_files_properties(kmp_import.cpp PROPERTIES COMPILE_FLAGS "${LIBOMP_CONFIGURED_CXXFLAGS}")
-  set(LIBOMP_IMP_LIB_FILE ${LIBOMP_LIB_NAME}${CMAKE_STATIC_LIBRARY_SUFFIX})
+  set(LIBOMP_IMP_LIB_FILE ${LIBOMP_LIB_NAME}${CMAKE_IMPORT_LIBRARY_SUFFIX})
   set(LIBOMP_GENERATED_IMP_LIB_FILENAME ${LIBOMP_LIB_FILE}${CMAKE_STATIC_LIBRARY_SUFFIX})
   set_target_properties(omp PROPERTIES
     VERSION ${LIBOMP_VERSION_MAJOR}.${LIBOMP_VERSION_MINOR} # uses /version flag
     IMPORT_PREFIX "" IMPORT_SUFFIX "" # control generated import library name when building omp
     ARCHIVE_OUTPUT_NAME ${LIBOMP_GENERATED_IMP_LIB_FILENAME}
   )
 
   if(MSVC)