Introduce separate compiler flag for unrestricted builder inference

compiler/testData/cli/js/jsExtraHelp.out

@@ -84,6 +84,8 @@ where advanced options include:
   -Xskip-metadata-version-check Allow to load classes with bad metadata version and pre-release classes
   -Xskip-prerelease-check    Allow to load pre-release classes
   -Xsuppress-version-warnings Suppress warnings about outdated, inconsistent or experimental language or API versions
+  -Xunrestricted-builder-inference
+                             Eliminate builder inference restrictions like allowance of returning type variables of a builder inference call
   -Xuse-experimental=<fq.name> Enable, but don't propagate usages of experimental API for marker annotation with the given fully qualified name
   -Xuse-fir                  Compile using Front-end IR. Warning: this feature is far from being production-ready
   -Xuse-fir-extended-checkers Use extended analysis mode based on Front-end IR. Warning: this feature is far from being production-ready

compiler/testData/cli/jvm/extraHelp.out

@@ -194,6 +194,8 @@ where advanced options include:
   -Xskip-metadata-version-check Allow to load classes with bad metadata version and pre-release classes
   -Xskip-prerelease-check    Allow to load pre-release classes
   -Xsuppress-version-warnings Suppress warnings about outdated, inconsistent or experimental language or API versions
+  -Xunrestricted-builder-inference
+                             Eliminate builder inference restrictions like allowance of returning type variables of a builder inference call
   -Xuse-experimental=<fq.name> Enable, but don't propagate usages of experimental API for marker annotation with the given fully qualified name
   -Xuse-fir                  Compile using Front-end IR. Warning: this feature is far from being production-ready
   -Xuse-fir-extended-checkers Use extended analysis mode based on Front-end IR. Warning: this feature is far from being production-ready

compiler/testData/cli/jvm/unrestrictedBuilderInference.args

@@ -0,0 +1,5 @@
+$TESTDATA_DIR$/unrestrictedBuilderInference.kt
+-d
+$TEMP_DIR$
+-Xunrestricted-builder-inference
+-Xopt-in=kotlin.RequiresOptIn

compiler/testData/cli/jvm/unrestrictedBuilderInference.kt

@@ -0,0 +1,6 @@
+@OptIn(ExperimentalStdlibApi::class)
+fun main() {
+    val x = buildMap {
+        val y = put(1, "")
+    }
+}

compiler/testData/cli/jvm/unrestrictedBuilderInference.out

@@ -0,0 +1,7 @@
+compiler/testData/cli/jvm/unrestrictedBuilderInference.kt:3:9: warning: variable 'x' is never used
+    val x = buildMap {
+        ^
+compiler/testData/cli/jvm/unrestrictedBuilderInference.kt:4:13: warning: variable 'y' is never used
+        val y = put(1, "")
+            ^
+OK