Fix "Incorrect CachedValue use" exception in RsFile

[vlad20012]

The bug was introduced in #5690

Stacktrace

java.lang.Throwable: Incorrect CachedValue use: same CV with different captured context, this can cause unstable results and invalid PSI access.
Field this$0 in class org.rust.lang.core.psi.RsFile$declarations$1 has non-equivalent values:
FILE (org.rust.lang.core.psi.RsFile) and
FILE (org.rust.lang.core.psi.RsFile)
Either make `equals()` hold for these values, or avoid this dependency, e.g. by extracting CV provider into a static method.
  at com.intellij.openapi.diagnostic.Logger.error(Logger.java:143)
  at com.intellij.util.CachedValueStabilityChecker.complain(CachedValueStabilityChecker.java:163)
  at com.intellij.util.CachedValueStabilityChecker.checkFieldEquivalence(CachedValueStabilityChecker.java:130)
  at com.intellij.util.CachedValueStabilityChecker.checkFieldEquivalence(CachedValueStabilityChecker.java:126)
  at com.intellij.util.CachedValueStabilityChecker.checkProvidersEquivalent(CachedValueStabilityChecker.java:81)
  at com.intellij.util.CachedValuesManagerImpl.getCachedValue(CachedValuesManagerImpl.java:71)
  at com.intellij.psi.util.CachedValuesManager.getCachedValue(CachedValuesManager.java:150)
  at org.rust.lang.core.psi.RsFile.getDeclarations(RsFile.kt:205)
  at org.rust.lang.core.psi.RsFile.getDeclaration(RsFile.kt:190)
  ...

bors r+

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bors r+

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