configs/configupgrade: Preserve comments on items in object exprs

The expression upgrade functionality mostly ignores comments because in
the old language the syntax prevented comments from appearing in the
middle of expressions, but there was one exception: object expressions.

Because HCL 1 used ObjectType both for blocks and for object expressions,
that is the one situation where something we consider to be an expression
could have inline attached comments in the old language.

We migrate these here so we don't lose these comments that don't appear
anywhere else. Other comments get gathered up into a general comments
set maintained inside the analysis object and so will be printed out as
required _between_ expressions, just as they did before.

configs/configupgrade/test-fixtures/valid/noop/input/resources.tf

@@ -1,5 +1,10 @@
 
 resource "test_instance" "example" {
+  tags = {
+    # Thingy thing
+    name = "foo bar baz" # this is a terrible name
+  }
+
   connection {
     host = "127.0.0.1"
   }

configs/configupgrade/test-fixtures/valid/noop/want/resources.tf

@@ -1,5 +1,10 @@
 
 resource "test_instance" "example" {
+  tags = {
+    # Thingy thing
+    name = "foo bar baz" # this is a terrible name
+  }
+
   connection {
     host = "127.0.0.1"
   }

configs/configupgrade/upgrade_expr.go

@@ -192,9 +192,22 @@ Value:
 			diags = diags.Append(moreDiags)
 			valueSrc, moreDiags := upgradeExpr(item.Val, filename, interp, an)
 			diags = diags.Append(moreDiags)
+			if item.LeadComment != nil {
+				for _, c := range item.LeadComment.List {
+					buf.WriteString(c.Text)
+					buf.WriteByte('\n')
+				}
+			}
+
 			buf.Write(keySrc)
 			buf.WriteString(" = ")
 			buf.Write(valueSrc)
+			if item.LineComment != nil {
+				for _, c := range item.LineComment.List {
+					buf.WriteByte(' ')
+					buf.WriteString(c.Text)
+				}
+			}
 			buf.WriteString("\n")
 		}
 		buf.WriteString("}")