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/stubs/Contracts.expected

@@ -0,0 +1,23 @@
+PsiJetFileStubImpl[package=test]
+  FILE_ANNOTATION_LIST
+    ANNOTATION_ENTRY[hasValueArguments=true, shortName=Suppress]
+      ANNOTATION_TARGET[useSiteTarget=FILE]
+      CONSTRUCTOR_CALLEE
+        TYPE_REFERENCE
+          USER_TYPE
+            REFERENCE_EXPRESSION[referencedName=Suppress]
+  PACKAGE_DIRECTIVE
+    REFERENCE_EXPRESSION[referencedName=test]
+  IMPORT_LIST
+    IMPORT_DIRECTIVE[importedFqName=kotlin.internal.contracts, isAllUnder=true, isValid=true]
+      DOT_QUALIFIED_EXPRESSION
+        DOT_QUALIFIED_EXPRESSION
+          REFERENCE_EXPRESSION[referencedName=kotlin]
+          REFERENCE_EXPRESSION[referencedName=internal]
+        REFERENCE_EXPRESSION[referencedName=contracts]
+  FUN[fqName=test.myRequire, hasBlockBody=true, hasBody=true, hasTypeParameterListBeforeFunctionName=false, isExtension=false, isTopLevel=true, mayHaveContract=true, name=myRequire]
+    VALUE_PARAMETER_LIST
+      VALUE_PARAMETER[fqName=null, hasDefaultValue=false, hasValOrVar=false, isMutable=false, name=x]
+        TYPE_REFERENCE
+          USER_TYPE
+            REFERENCE_EXPRESSION[referencedName=Boolean]