[MLIR][Doc] Also print `summary`s for passes on a newline

[MLIR][Doc] Also print `summary`s for passes on a newline

This patch is improves upon https://reviews.llvm.org/D152621. There, I pointed out some issues with D152621, which I'll repeat here.

> Passes use a different logic for generating the documentation; which I didn't update to be in-line with this change.

Fixed by defining and using `mlir::tblgen::emitSummary`. This is now used in `OpDocGen.cpp` and `PassDocGen.cpp`.

Note that the passes documentation currently prints the summary behind the pass argument. For example:

```
#### -arm-neon-2d-to-intr: Convert Arm NEON structured ops to intrinsics
```
at https://mlir.llvm.org/docs/Passes/#-promote-buffers-to-stack-promotes-heap-based-allocations-to-automatically-managed-stack-based-allocations.

This currently differs from how the summary is printed for Ops. For example:

```
#### amdgpu.lds_barrier (::mlir::amdgpu::LDSBarrierOp) ¶

**Summary:** _Barrier that includes a wait for LDS memory operations._
```

at https://mlir.llvm.org/docs/Dialects/AMDGPU/#amdgpulds_barrier-mliramdgpuldsbarrierop.

The changes in this patch ensure that:

1. The summary is always printed on a new line.
2. The summary is always printed in italic.
3. The summary always starts with a capital letter.

I've dropped the `**Summary:**`, which was introduced in D152621, because only italicization should be already clear enough.

> `amx.tdpbssd` shows **Summary:** __ meaning that apparently hasSummary does not guarantee a non-empty summary.

This is fixed by double-checking `!summary.empty()`, because the following code

```cpp
void mlir::tblgen::emitSummary(StringRef summary, raw_ostream &os) {
  if (!summary.empty()) {
    char first = std::toupper(summary.front());
    llvm::StringRef rest = summary.drop_front();
    os << "\n_" << first << rest << "_\n\n";
  } else {
    os << "\n_" << "foo" << "_\n\n";
  }
}
```
generates the following Markdown:
```
### `amx.tdpbssd` (::mlir::amx::x86_amx_tdpbssd)

_foo_
```
in `tools/mlir/docs/Dialects/AMX.md`.

> Summary fields containing * cancel the italicization, so the * should probably be escaped to solve this. EDIT: Nope. This is because mlir-www runs Hugo 0.80 whereas 0.111 correctly parses _Raw Buffer Floating-point Atomic Add (MI-* only)_ as an italicized string.

This will be fixed by https://github.com/llvm/mlir-www/pull/152.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D152648