Add fallback flag for legacy var smart casts after try #KT-21377 Fixed

compiler/testData/cli/js/jsExtraHelp.out

@@ -7,6 +7,7 @@ where advanced options include:
   -Xcoroutines={enable|warn|error}
                              Enable coroutines or report warnings or errors on declarations and use sites of 'suspend' modifier
   -Xintellij-plugin-root=<path> Path to the kotlin-compiler.jar or directory where IntelliJ configuration files can be found
+  -Xlegacy-smart-cast-after-try Allow var smart casts despite assignment in try block
   -Xmulti-platform           Enable experimental language support for multi-platform projects
   -Xnew-inference            Enable new experimental generic type inference algorithm
   -Xno-check-actual          Do not check presence of 'actual' modifier in multi-platform projects

compiler/testData/cli/jvm/extraHelp.out

@@ -34,6 +34,7 @@ where advanced options include:
   -Xcoroutines={enable|warn|error}
                              Enable coroutines or report warnings or errors on declarations and use sites of 'suspend' modifier
   -Xintellij-plugin-root=<path> Path to the kotlin-compiler.jar or directory where IntelliJ configuration files can be found
+  -Xlegacy-smart-cast-after-try Allow var smart casts despite assignment in try block
   -Xmulti-platform           Enable experimental language support for multi-platform projects
   -Xnew-inference            Enable new experimental generic type inference algorithm
   -Xno-check-actual          Do not check presence of 'actual' modifier in multi-platform projects

compiler/testData/cli/jvm/legacySmartCastsAfterTry.args

@@ -0,0 +1,4 @@
+$TESTDATA_DIR$/legacySmartCastsAfterTry.kt
+-d
+$TEMP_DIR$
+-Xlegacy-smart-cast-after-try

compiler/testData/cli/jvm/legacySmartCastsAfterTry.kt

@@ -0,0 +1,9 @@
+fun foo() {
+    var some: String? = null
+    some = "alpha"
+    try {
+        some = "omega"
+    }
+    catch (e: Exception) {}
+    some.length
+}

compiler/testData/cli/jvm/legacySmartCastsAfterTry.out

@@ -0,0 +1,4 @@
+compiler/testData/cli/jvm/legacySmartCastsAfterTry.kt:2:25: warning: variable 'some' initializer is redundant
+    var some: String? = null
+                        ^
+OK