[mlir] Fix INSTALL_INTERFACE path for MLIRSparseTensorEnums

[mgorny]

Summary

Fix INSTALL_INTERFACE path for exported MLIRSparseTensorEnums target
to include the include directory. Otherwise, CMake attempts to find it
relatively to CMAKE_INSTALL_PREFIX rather than the include directory,
resulting in flang failing to configure with the following error:

CMake Error at /usr/lib/llvm/16/lib64/cmake/llvm/AddLLVM.cmake:581 (add_library):
  Cannot find source file:

    /usr/lib/llvm/16/mlir/Dialect/SparseTensor/IR/Enums.h

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .mpp .m .M .mm .ixx .cppm .h
  .hh .h++ .hm .hpp .hxx .in .txx .f .F .for .f77 .f90 .f95 .f03 .hip .ispc
Call Stack (most recent call first):
  cmake/modules/AddFlang.cmake:64 (llvm_add_library)
  lib/Lower/CMakeLists.txt:3 (add_flang_library)

[mgorny]

Thanks!

mlir/lib/Dialect/SparseTensor/IR/CMakeLists.txt

 # INSTALL_INTERFACE generator expressions to avoid paths being interpreted
 # as absolute; for more details, see <https://stackoverflow.com/a/62465051>.
 # Unfortunately BUILD_INTERFACE and INSTALL_INTERFACE require their file
 # paths to be relative to different things, hence why we use a foreach loop
 # to iterate over the sources (in case we ever need to have more than one).
 foreach(hdr ${MLIRSparseTensorEnums_hdrs})
   target_sources(MLIRSparseTensorEnums INTERFACE
     $<BUILD_INTERFACE:${MLIR_MAIN_INCLUDE_DIR}/${hdr}>
-    $<INSTALL_INTERFACE:${hdr}>)
+    $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/${hdr}>)
 endforeach()
 # The `add_mlir_library_install` is required for other libraries to
 # depend on this one, but the conditional itself and the phony target
 # are copypasta from `add_mlir_library`.  Afaict (wrengr), the version
 # in `add_mlir_library` is to prevent `add_mlir_library_install` from
 # raising additional errors whenever the underlying call to `add_llvm_library`
 # fails.  However, since we are using `add_library` directly, I'm not
 # sure if the conditional is helpful/required here or not.