[MLIR][Timing] Nested pipeline is displayed at top level and show negative Rest field when using -mlir-timing

When we run dynamic pass pipeline via `LogicalResult pass::runPipeline(OpPassManager &pipeline, Operation *op)` within a pass, the timing infra will print the pass's timing at the top level, instead of nesting it. Meanwhile, we can find a negative Rest field in the timing report, which might be due to the nested pass pipeline are double counted.

The minimal example to reproduce is `inliner` pass which nest itself with a `Canonicalizer` pass. We can see that the `Canonicalizer` pass was shown at top level and there is a -5.5% Rest field.

```
build/install/bin/mlir-opt -inline mlir/test/Transforms/inlining-repeated-use.mlir -mlir-timing
===-------------------------------------------------------------------------===
... Execution time report ...
===-------------------------------------------------------------------------===
Total Execution Time: 0.0009 seconds

----Wall Time---- ----Name----
0.0003 ( 40.1%) Parser
0.0003 ( 37.5%) Inliner
0.0000 ( 1.3%) (A) CallGraph
0.0001 ( 7.9%) 'func.func' Pipeline
0.0001 ( 7.7%) Canonicalizer
0.0002 ( 19.9%) Output
-0.0000 ( -5.5%) Rest
0.0009 (100.0%) Total
```

From the source code [mlir/lib/Pass/PassTiming.cpp#L60-L67](https://github.com/llvm/llvm-project/blob/main/mlir/lib/Pass/PassTiming.cpp#L60-L67), it seems the nested pass does not have a `parentTimer` so they will fallback to the root timer and lead to this phenomenon. Is this by design or need a fix?

Start with mlir/lib/Pass/PassTiming.cpp#L60-L67 and reproduce the report using mlir/test/Transforms/inlining-repeated-use.mlir with -mlir-timing. Trace how runPipeline handles the nested Inliner and Canonicalizer timers; done means nested passes appear under their parent and the report no longer shows a negative Rest field.

Written by the indexing model from the issue text.