[mlir] NFC: Rename index_t to index_type

mlir currently fails to build on Solaris:

  /vol/llvm/src/llvm-project/dist/mlir/lib/Conversion/VectorToLoops/ConvertVectorToLoops.cpp:78:20: error: reference to 'index_t' is ambiguous
    IndexHandle zero(index_t(0)), one(index_t(1));
                     ^
  /usr/include/sys/types.h:103:16: note: candidate found by name lookup is 'index_t'
  typedef short           index_t;
                          ^
  /vol/llvm/src/llvm-project/dist/mlir/include/mlir/EDSC/Builders.h:27:8: note: candidate found by name lookup is 'mlir::edsc::index_t'
  struct index_t {
         ^

and many more.

Given that POSIX reserves all identifiers ending in `_t` 2.2.2 The Name Space <https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html>, it seems
quite unwise to use such identifiers in user code, even more so without a distinguished
prefix.

The following patch fixes this by renaming `index_t` to `index_type`.
cases.

Tested on `amd64-pc-solaris2.11` and `sparcv9-sun-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D72619