Support contracts in PartialBodyResolveFilter

Previously, PartialBodyResolveFilter didn't know about contracts and was
filtering out calls of such functions, leading to unstable completion in
cases like that:

fun test(x: Any?) {
    require(x is String)
    x.<caret>
}

However, PartialBodyResolveFilter works by pure PSI, while to determine
if function has a contract we have to resolve it.

To solve it, we do something very similar to what has been done with
Nothin-returning functions: introduce
KotlinProbablyContractedFunctionShortNameIndex, which collects all
function which *may* have a contract during indexing.

^KT-25275 Fixed

idea/testData/resolve/partialBodyResolve/ContractFromSource.completion

@@ -0,0 +1,22 @@
+Resolve target: value-parameter x: kotlin.Any? smart-cast to kotlin.String
+----------------------------------------------
+// COMPILER_ARGUMENTS: -XXLanguage:+AllowContractsForCustomFunctions -XXLanguage:+UseReturnsEffect
+@file:Suppress("INVISIBLE_MEMBER")
+package test
+
+import kotlin.internal.contracts.*
+
+fun myRequire(x: Boolean) {
+    contract {
+        returns() implies x
+    }
+}
+
+fun testContractFromSource(x: Any?, y: Any?) {
+    myRequire(x is String)
+    myRequire(y is String)
+
+    <caret>x.length
+
+    /* STATEMENT DELETED: myRequire(x is Int) */
+}