[Bug] [iOS] Editor control _placeholderLabel LayoutConstraints are broken

[dmitrymal]

Description

On iOS the layout constrains for the _placeholderLabel in the EditorRenderer are broken:

2020-08-23 00:26:33.317501-0500 FormsTest.iOS[98021:4059822] [LayoutConstraints] Unable to simultaneously satisfy constraints.
Probably at least one of the constraints in the following list is one you don't want.
Try this:
(1) look at each constraint and try to figure out which you don't expect;
(2) find the code that added the unwanted constraint or constraints and fix it.
(Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints)
(
"<NSAutoresizingMaskLayoutConstraint:0x600002f96f80 h=--& v=--& UILabel:0x7f985ca25020'Test Editor'.minX == 0 (active, names: '|':FormsTest_iOS_EditorRendererBase_1_FormsUITextView:0x7f985db2a600'Test Editor' )>",
"<NSLayoutConstraint:0x600002fe7bb0 H:|-(5)-[UILabel:0x7f985ca25020'Type in!!!'] (active, names: '|':FormsTest_iOS_EditorRendererBase_1_FormsUITextView:0x7f985db2a600'Type in!!!' )>"
)

Steps to Reproduce

1. Add Editor to the xaml page (e.g. MainPage)
2. Run the iOS project in the debugger
3. Navigate to the main page. You will see the above message in the Application output window

Expected Behavior

No iOS constraint issues

Actual Behavior

iOS is complaining about some Layout Constraints not being satisfied.

Basic Information

• Version with issue:
  4.8.0.1269

• Last known good version:
  Not sure, but I traced it to this PR:
  063b245#diff-71148fd2814c3d17185db42cc58913ff
  Specifically, this line:
  063b245#diff-71148fd2814c3d17185db42cc58913ffR119
  _placeholderLabel.TranslatesAutoresizingMaskIntoConstraints = true;
  I don't know the reason why this line was changed from false to true, but when I change it back to false, I got no Constraint issues.

• IDE:
  VS for Mac

• Platform Target Frameworks:

  • iOS: 13.6

• Nuget Packages:

Workaround

I created a custom Editor Renderer and replace it with the old code.

[jsuarezruiz]

This other PR is related with this one: #11889