Index a method body in TreePathCacher after repeated path lookup misses

[wmdietl]

Fixes #2260.

Problem. During dataflow, AnnotatedTypeFactory.getPath searches from the path of the method body being analyzed. TreePathCacher caches only the path to the target, so every lookup for a node in the body rescans the statements before it. Finding the defaulting scope (QualifierDefaults.nearestEnclosingExceptLocal) of every node therefore makes a method body quadratic in its number of statements. JFR on the shape of checker/jtreg/nullness/Issue1438.java (4000 fields, one static method with 4000 map.put("kN", vN) calls) had 169 of 590 samples under nearestEnclosingExceptLocal.

Fix. After 32 consecutive getPath(TreePath, Tree) misses with the same leaf, TreePathCacher records a child-to-parent map of that leaf's subtree and answers later lookups under it by walking the parent chain. One map is kept, replaced when another leaf qualifies, and dropped by clear(). The path returned is the one the scan finds, including for trees that occur twice (the shared type tree of Integer a, b;): the map keeps the first occurrence in scan order.

Measurements. Nullness Checker, -proc:only, JDK 21, median of 3 interleaved runs, N calls and N fields:

N	master	branch
2000	8.08 s	7.57 s
4000	15.14 s	12.37 s
8000	33.99 s	25.87 s

nearestEnclosingExceptLocal falls from 169 of 590 to 3 of 448 samples at N=4000. Real code is unaffected: javacutil and dataflow sources (209 files), 5 interleaved runs, 21.9 s vs 22.2 s, within noise. Diagnostics are identical on every workload.

Tried and rejected (recorded in docs/developer/performance-notes.md): registering the CFG builder's own paths in the cacher (they skip tree levels such as MemberSelectTree and include artificial trees, so they differ from the real paths), and short-circuiting nearestEnclosingExceptLocal for declaration trees (it moves the cost to InitializationAnnotatedTypeFactory.isInitializedInTargetStore and was slower overall).

Tests. New TreePathCacherTest.manyLookupsUnderOneLeafMatchJdk compares against TreePath.getPath for every tree, cold and cached, in forward and reverse order, including trees outside the body and absent from the unit. ./gradlew :framework:test, the whole ./gradlew :checker:test (91 classes) and ./gradlew :checker:jtregTests pass.

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