Show only unique diagnostics in psi checker (KT-35578)

MissingDependencyClassChecker.collectDiagnostics now show only unique diagnostics

As per-file analyzer trace used in checker delegates to resolve session trace, diagnostics might be duplicated because of race condition:
 1. If a non-checker thread performs analyze first, diagnostics for global elements will be stored in the resolve session trace only once.
 2 If the checker threads comes to the analyze first, diagnostics will be stored in the local trace, and after that might be duplicated in the resolve session trace by other analyzers.

 #KT-35578 Fixed

idea/testData/codeInsight/outOfBlock/PropNullType_Initializer_If_Fun.kt

@@ -1,7 +1,6 @@
 // OUT_OF_CODE_BLOCK: FALSE
 // ERROR: This variable must either have a type annotation or be initialized
 // ERROR: Type mismatch: inferred type is Unit but Int? was expected
-// ERROR: Type mismatch: inferred type is Unit but Int? was expected
 
 val test: Int? = if (true) {
     fun test() {