Intern unresolved symlink targets (#31122)

`UnresolvedSymlinkAction` interns its destination string in the constructor. Actions with the same destination share string storage without changing the raw target value, action key, or deferred `PathFragment` conversion.

One matched pair of partial monorepo analyses retained 3.27 million fewer strings and backing arrays, accounting for about 664 MB in the class histogram. Reported after-GC heap was 676 MB lower; analysis time and peak RSS were essentially unchanged. This is a single comparison, not a repeatability claim.

Validation on upstream master: all six `UnresolvedSymlinkActionTest` cases pass, covering action keys, symlink creation, and serialization.

RELNOTES: None

-zbarskybot

Closes #31122

COPYBARA_INTEGRATE_REVIEW=https://github.com/bazelbuild/bazel/pull/31122 from dzbarsky:codex/symlink-targets-upstream 9f444949636d5a953d7ad10c3bc8f978f79988ce
PiperOrigin-RevId: 981263379
Change-Id: I0e825a11a001568b5c19e83b81c8e70959bdc905

src/main/java/com/google/devtools/build/lib/analysis/starlark/UnresolvedSymlinkAction.java

diff --git a/src/main/java/com/google/devtools/build/lib/analysis/starlark/UnresolvedSymlinkAction.java b/src/main/java/com/google/devtools/build/lib/analysis/starlark/UnresolvedSymlinkAction.java
index c0d81f7..43ab770 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/starlark/UnresolvedSymlinkAction.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/starlark/UnresolvedSymlinkAction.java
@@ -58,7 +58,7 @@
       SymlinkTargetType targetType,
       String progressMessage) {
     super(owner, NestedSetBuilder.emptySet(Order.STABLE_ORDER), ImmutableSet.of(primaryOutput));
-    this.target = target;
+    this.target = target.intern();
     this.targetType = targetType;
     this.progressMessage = progressMessage;
   }