Introduce 'mayHaveContract'-flag in stubs

This is needed for further commit, which supports contracts-based
smartcasts in partial body resolve mode.

NB: Stubs can be built from 3 sources:
- source code (contract presence can be checked by PSI)
- binary data (contract presence can be checked by Kotlin Metadata)
- decompiled sources

The last case is a bit of a headache, because usually bodies are omitted
in decompiled sources. To workaround it, we have to inject stubbed
contract-call in the body.

idea/testData/decompiler/stubBuilder/Contracts/Contracts.kt

@@ -0,0 +1,12 @@
+// JVM_FILE_NAME: ContractsKt
+
+@file:Suppress("INVISIBLE_MEMBER")
+package test
+
+import kotlin.internal.contracts.*
+
+fun myRequire(x: Boolean) {
+    contract {
+        returns(true) implies (x)
+    }
+}

idea/testData/decompiler/stubBuilder/Contracts/Contracts.txt

@@ -0,0 +1,18 @@
+PsiJetFileStubImpl[package=test]
+  PACKAGE_DIRECTIVE
+    REFERENCE_EXPRESSION[referencedName=test]
+  IMPORT_LIST
+  FUN[fqName=test.myRequire, hasBlockBody=true, hasBody=true, hasTypeParameterListBeforeFunctionName=false, isExtension=false, isTopLevel=true, mayHaveContract=true, name=myRequire]
+    MODIFIER_LIST[public]
+    VALUE_PARAMETER_LIST
+      VALUE_PARAMETER[fqName=null, hasDefaultValue=false, hasValOrVar=false, isMutable=false, name=x]
+        TYPE_REFERENCE
+          USER_TYPE
+            USER_TYPE
+              REFERENCE_EXPRESSION[referencedName=kotlin]
+            REFERENCE_EXPRESSION[referencedName=Boolean]
+    TYPE_REFERENCE
+      USER_TYPE
+        USER_TYPE
+          REFERENCE_EXPRESSION[referencedName=kotlin]
+        REFERENCE_EXPRESSION[referencedName=Unit]