configs/configupgrade: Silently ignore and trim .% .# in ignore_changes

Prior to Terraform 0.12, ignore_changes was implemented in a
flatmap-oriented fashion and so users found that they could (and in fact,
were often forced to) use the internal .% and .# suffixes flatmap uses to
ignore changes to the number of elements in a list or map.

Terraform 0.12 no longer uses that representation, so we'll interpret
ignoring changes to the length as ignoring changes to the entire
collection. While this is not a totally-equivalent change, in practice
this pattern was most often used in conjunction with specific keys from a
map in order to _effectively_ ignore the entire map, even though Terraform
didn't really support that.

configs/configupgrade/test-fixtures/valid/ignore-changes-flatmap-colls/input/ignore-changes-flatmap-colls.tf

@@ -0,0 +1,8 @@
+resource "test_instance" "foo" {
+  lifecycle {
+    ignore_changes = [
+      "a.%",
+      "b.#",
+    ]
+  }
+}

configs/configupgrade/test-fixtures/valid/ignore-changes-flatmap-colls/want/ignore-changes-flatmap-colls.tf

@@ -0,0 +1,8 @@
+resource "test_instance" "foo" {
+  lifecycle {
+    ignore_changes = [
+      a,
+      b,
+    ]
+  }
+}

configs/configupgrade/test-fixtures/valid/ignore-changes-flatmap-colls/want/versions.tf

@@ -0,0 +1,3 @@
+terraform {
+  required_version = ">= 0.12"
+}

configs/configupgrade/upgrade_expr.go

@@ -560,6 +560,14 @@ func upgradeHeredocBody(buf *bytes.Buffer, val *hilast.Output, filename string,
 func upgradeTraversalExpr(val interface{}, filename string, an *analysis) ([]byte, tfdiags.Diagnostics) {
 	if lit, ok := val.(*hcl1ast.LiteralType); ok && lit.Token.Type == hcl1token.STRING {
 		trStr := lit.Token.Value().(string)
+		if strings.HasSuffix(trStr, ".%") || strings.HasSuffix(trStr, ".#") {
+			// Terraform 0.11 would often not validate traversals given in
+			// strings and so users would get away with this sort of
+			// flatmap-implementation-detail reference, particularly inside
+			// ignore_changes. We'll just trim these off to tolerate it,
+			// rather than failing below in ParseTraversalAbs.
+			trStr = trStr[:len(trStr)-2]
+		}
 		trSrc := []byte(trStr)
 		_, trDiags := hcl2syntax.ParseTraversalAbs(trSrc, "", hcl2.Pos{})
 		if !trDiags.HasErrors() {