[LL API] Analyze code fragment in a separate 'FirSession'

Before, `KtCodeFragment`/`FirCodeFragment` was analyzed as a part of
its context `KtModule`. This has the following complications:

- In non-source sessions, diagnostic reporting is globally disabled.
  For code fragments, however, checking the code before passing it to
  the backend is essential.

- Special treatment for call ambiguities in libraries
  (`LLLibraryScopeAwareCallConflictResolverFactory`) becomes complicated
  as the conflict resolver has to be applied to a library module.

- `KtCodeFragment`s usually have a shorter lifetime than their own
  context. Caching may potentially be implemented differently for them.

^KT-61783 Fixed

analysis/analysis-api/testData/components/compilerFacility/compilation/codeFragments/capturing/objectFunction.ir.txt

@@ -9,4 +9,4 @@ MODULE_FRAGMENT
         EXPRESSION_BODY
           BLOCK type=kotlin.Unit origin=null
             CALL 'public final fun call (): kotlin.Unit declared in <root>.Foo' type=kotlin.Unit origin=null
-              $this: GET_OBJECT 'CLASS OBJECT name:Foo modality:FINAL visibility:public superTypes:[kotlin.Any]' type=<root>.Foo
+              $this: GET_OBJECT 'CLASS IR_EXTERNAL_DECLARATION_STUB OBJECT name:Foo modality:FINAL visibility:public superTypes:[kotlin.Any]' type=<root>.Foo

analysis/analysis-api/testData/components/compilerFacility/compilation/codeFragments/capturing/outerClassMutatedPrivate.fragment.kt

@@ -1 +1,2 @@
+@Suppress("INVISIBLE_SETTER")
 a = "bar"