FIR2IR: add test about coercion-to-Unit for nested when

compiler/testData/ir/irText/firProblems/coercionToUnitForNestedWhen.kt

@@ -0,0 +1,17 @@
+// WITH_RUNTIME
+// FULL_JDK
+import java.io.Reader
+import java.io.StringReader
+
+private const val BACKSLASH = '\\'
+
+private fun Reader.nextChar(): Char? =
+    read().takeUnless { it == -1 }?.toChar()
+
+fun Reader.consumeRestOfQuotedSequence(sb: StringBuilder, quote: Char) {
+    var ch = nextChar()
+    while (ch != null && ch != quote) {
+        if (ch == BACKSLASH) nextChar()?.let { sb.append(it) } else sb.append(ch)
+        ch = nextChar()
+    }
+}