KT-7715 Highlight var's that can be replaced by val's

#KT-7715 Fixed

idea/testData/inspections/canBeVal/alwaysAssigned.kt

@@ -0,0 +1,6 @@
+fun foo(p: Int) {
+    var v1: Int
+    var v2: Int = 0
+    if (p > 0) v1 = 1 else v1 = 2
+    v2 = 1
+}

idea/testData/inspections/canBeVal/assignedInLoop.kt

@@ -0,0 +1,6 @@
+fun foo(p: Int) {
+    var v: Int
+    for (i in 1..10) {
+        v = i
+    }
+}

idea/testData/inspections/canBeVal/assignedTwice.kt

@@ -0,0 +1,5 @@
+fun foo(p: Int) {
+    var v: Int
+    v = 0
+    if (p > 0) v = 1
+}

idea/testData/inspections/canBeVal/desctructuringDeclaration1.kt

@@ -0,0 +1,6 @@
+fun foo(p: Int) {
+    var (v1, v2) = getPair()
+    print(v1)
+}
+
+fun getPair(): Pair<Int, String> = 1 to ""

idea/testData/inspections/canBeVal/desctructuringDeclaration2.kt

@@ -0,0 +1,7 @@
+fun foo(p: Int) {
+    var (v1, v2) = getPair()
+    print(v1)
+    v2 = ""
+}
+
+fun getPair(): Pair<Int, String> = 1 to ""

idea/testData/inspections/canBeVal/desctructuringDeclarationUnused.kt

@@ -0,0 +1,5 @@
+fun foo(p: Int) {
+    var (v1, v2) = getPair()
+}
+
+fun getPair(): Pair<Int, String> = 1 to ""

idea/testData/inspections/canBeVal/inspectionData/expected.xml

@@ -0,0 +1,55 @@
+<problems>
+    <problem>
+        <file>withInitializer.kt</file>
+        <line>4</line>
+        <module>light_idea_test_case</module>
+        <entry_point TYPE="file" FQNAME="withInitializer.kt" />
+        <problem_class severity="WARNING" attribute_key="WARNING_ATTRIBUTES">Local 'var' can be declared as 'val'</problem_class>
+        <description>Can be declared as 'val'</description>
+    </problem>
+
+    <problem>
+        <file>alwaysAssigned.kt</file>
+        <line>2</line>
+        <module>light_idea_test_case</module>
+        <entry_point TYPE="file" FQNAME="alwaysAssigned.kt" />
+        <problem_class severity="WARNING" attribute_key="WARNING_ATTRIBUTES">Local 'var' can be declared as 'val'</problem_class>
+        <description>Can be declared as 'val'</description>
+    </problem>
+
+    <problem>
+        <file>notAssignedWhenNotUsed.kt</file>
+        <line>2</line>
+        <module>light_idea_test_case</module>
+        <entry_point TYPE="file" FQNAME="notAssignedWhenNotUsed.kt" />
+        <problem_class severity="WARNING" attribute_key="WARNING_ATTRIBUTES">Local 'var' can be declared as 'val'</problem_class>
+        <description>Can be declared as 'val'</description>
+    </problem>
+
+    <problem>
+        <file>twoVariables.kt</file>
+        <line>2</line>
+        <module>light_idea_test_case</module>
+        <entry_point TYPE="file" FQNAME="twoVariables.kt" />
+        <problem_class severity="WARNING" attribute_key="WARNING_ATTRIBUTES">Local 'var' can be declared as 'val'</problem_class>
+        <description>Can be declared as 'val'</description>
+    </problem>
+
+    <problem>
+        <file>twoVariables.kt</file>
+        <line>3</line>
+        <module>light_idea_test_case</module>
+        <entry_point TYPE="file" FQNAME="twoVariables.kt" />
+        <problem_class severity="WARNING" attribute_key="WARNING_ATTRIBUTES">Local 'var' can be declared as 'val'</problem_class>
+        <description>Can be declared as 'val'</description>
+    </problem>
+
+    <problem>
+        <file>desctructuringDeclaration1.kt</file>
+        <line>2</line>
+        <module>light_idea_test_case</module>
+        <entry_point TYPE="file" FQNAME="desctructuringDeclaration1.kt" />
+        <problem_class severity="WARNING" attribute_key="WARNING_ATTRIBUTES">Local 'var' can be declared as 'val'</problem_class>
+        <description>Can be declared as 'val'</description>
+    </problem>
+</problems>

idea/testData/inspections/canBeVal/inspectionData/inspections.test

@@ -0,0 +1 @@
+// INSPECTION_CLASS: org.jetbrains.kotlin.idea.inspections.CanBeValInspection

idea/testData/inspections/canBeVal/notAssignedWhenNotUsed.kt

@@ -0,0 +1,7 @@
+fun foo(p: Int) {
+    var v: Int
+    if (p > 0) {
+        v = 1
+        print(v)
+    }
+}

idea/testData/inspections/canBeVal/onlyLocal.kt

@@ -0,0 +1,10 @@
+var global = 1
+
+class C {
+    var field = 2
+
+    fun foo() {
+        print(field)
+        print(global)
+    }
+}

idea/testData/inspections/canBeVal/onlyVar.kt

@@ -0,0 +1,4 @@
+fun foo() {
+    val v = 1
+    print(v)
+}

idea/testData/inspections/canBeVal/twoVariables.kt

@@ -0,0 +1,6 @@
+fun foo(p: Int) {
+    var v1: Int
+    var v2: Int
+    if (p > 0) v1 = 1 else v1 = 2
+    v2 = 1
+}

idea/testData/inspections/canBeVal/unused.kt

@@ -0,0 +1,3 @@
+fun foo() {
+    var v: Int
+}

idea/testData/inspections/canBeVal/unusedWithInitializer.kt

@@ -0,0 +1,3 @@
+fun foo() {
+    var v = 1
+}

idea/testData/inspections/canBeVal/withInitializer.kt

@@ -0,0 +1,8 @@
+fun foo() {
+    var v1 = 1
+    var v2 = 2
+    var v3 = 3
+    v1 = 1
+    v2++
+    print(v3)
+}