Clarify condition for block_given? desugar

Checking the `enclosingBlockParamName` was really just a proxy for
whether we were in a method or not.

Even for methods with no explicit block parameter, `buildMethod()`,
will add a synthetic `&<blk>`, so the if we're in a method,
`enclosingBlockParamName` is always set by the time it's checked here.

Author: amomchilov

ast/desugar/Desugar.cc

@@ -748,7 +748,7 @@ ExpressionPtr node2TreeImplBody(DesugarContext dctx, parser::Node *what) {
                     flags.isPrivateOk = true;
                 }
 
-                if (isCallToBlockGivenP(send, rec) && dctx.enclosingBlockParamName.exists()) {
+                if (isCallToBlockGivenP(send, rec) && dctx.enclosingMethodLoc.exists()) {
                     // Desugar:
                     //     def foo(&my_block)
                     //       x = block_given?

ast/desugar/PrismDesugar.cc

@@ -732,7 +732,7 @@ ExpressionPtr node2TreeImplBody(DesugarContext dctx, parser::Node *what) {
                     flags.isPrivateOk = true;
                 }
 
-                if (isCallToBlockGivenP(send, rec) && dctx.enclosingBlockParamName.exists()) {
+                if (isCallToBlockGivenP(send, rec) && dctx.enclosingMethodLoc.exists()) {
                     // Desugar:
                     //     def foo(&my_block)
                     //       x = block_given?

test/testdata/desugar/block_given.rb

@@ -52,3 +52,13 @@ def method_with_implicit_block_param()
   SomethingElse.block_given?
   block_given?("with parameter")
 end
+
+# Outside of a method, none of these shuold desugar to special calls to `block_given?`
+block_given?
+self.block_given?
+Kernel.block_given?
+::Kernel.block_given?
+block_given? { "with a block?!" }
+Object.block_given? # Normally not allowed: private method `block_given?' called for Object:Class
+SomethingElse.block_given?
+block_given?("with parameter")

test/testdata/desugar/block_given.rb.desugar-tree.exp

@@ -113,4 +113,22 @@ class <emptyTree><<C <root>>> < (::<todo sym>)
       <self>.block_given?("with parameter")
     end
   end
+
+  <self>.block_given?()
+
+  <self>.block_given?()
+
+  <emptyTree>::<C Kernel>.block_given?()
+
+  ::<root>::<C Kernel>.block_given?()
+
+  <self>.block_given?() do ||
+    "with a block?!"
+  end
+
+  <emptyTree>::<C Object>.block_given?()
+
+  <emptyTree>::<C SomethingElse>.block_given?()
+
+  <self>.block_given?("with parameter")
 end