[FIR] Fix K2 behavior according to RULES1

The compiler should only report diagnostics for
comparisons over builtins and identity-less types,
other incompatibilities should be reported
via inspections.

It's ok that in `equalityChecksOnIntegerTypes`
instead of `EQUALITY_NOT_APPLICABLE_WARNING` we get
`EQUALITY_NOT_APPLICABLE`, because
`ProperEqualityChecksInBuilderInferenceCalls`
is already active by default.

This change also replaces the notion of a representative superclass
with the least upper bound.
This makes complex types like
intersection/flexible transparent to
RULES1-based compatibility checks.
One way to look at it is to think
that this is an automatic way of handling
type parameters: automatic picking of
"interesting" bounds, and checking them against one another.

Note that `TypeIntersector.intersectTypes`
for `Int` and `T` where `T` is a type parameter
may return both `{Int & T}` or `null`
depending on `T`-s bounds. At the same time,
for type parameters `T` and `K` it will
always return `{T & K}`.

`ConeTypeIntersector.intersectTypes`, on the
other hand, will always return `{Int & T}`
irrespectively of the bounds. Meaning, the two
intersectors differ in corner cases.

`lowerBoundIfFlexible` call in `isLiterallyTypeParameter` is backed by
the `equalityOfFlexibleTypeParameters` test.

^KT-35134 #fixed-in-k2
^KT-22499 #fixed-in-k2
^KT-46383 #fixed-in-k2

compiler/fir/analysis-tests/testData/resolveWithStdlib/intellij/IntersectionWithJavaString.kt

@@ -1,8 +1,9 @@
 // FULL_JDK
 // ISSUE: KT-48113
+// STATUS: On a K2 technical meeting on Mar-27-2023 it was decided to report a warning
 // WITH_EXTENDED_CHECKERS
 
 fun collapse(path: String) {
     val result = (path as <!PLATFORM_CLASS_MAPPED_TO_KOTLIN!>java.lang.String<!>).replace("123", "456")
-    if (result !== path) {}
+    if (<!EQUALITY_NOT_APPLICABLE_WARNING!>result !== path<!>) {}
 }