Merge master to dev15.6 (dotnet#4156)

* fix resource name (dotnet#4151)

otherwise instead of expected `FSStrings.resources` will use `FSharp.Compiler.Service.netstandard.FSStrings.resources`

* Use ConcurrentDictionary in ImportMap (dotnet#4148)

* Fix IndexOutOfRangeException in check for providing completion (dotnet#4138)

* Fix IndexOutOfRange in check for providing completion:

* Add test

* Remove repeating arguments processing in IncrementalBuilder creation (dotnet#4124)

* Report builder creation warnings according to compiler args (dotnet#4125)

* Filter incremental builder creation errors according to compiler args

* Fix CompilationErrorLogger ignores WarsAsError options

* Add test for WarnAsError

* Cleanup

* Add more tests; cover WarnAsError-, no warnings at all

* Refactor tests

* Add test

Author: dotnet-bot

Completion/CompletionProvider.fs

@@ -85,10 +85,9 @@ type internal FSharpCompletionProvider
         else
             let triggerPosition = caretPosition - 1
             let triggerChar = sourceText.[triggerPosition]
-            let prevChar = sourceText.[triggerPosition - 1]
-            
+
             // do not trigger completion if it's not single dot, i.e. range expression
-            if not Settings.IntelliSense.ShowAfterCharIsTyped && prevChar = '.' then
+            if not Settings.IntelliSense.ShowAfterCharIsTyped && triggerPosition > 0 && sourceText.[triggerPosition - 1] = '.' then
                 false
             else
                 let documentId, filePath, defines = getInfo()