Verifies that the current implementation matches the latest
LDM decisions on nullable suppression and out variables

Closes #27522

Use default literal for parameters in symbol display

Add an inline-description portion to regex-completion to make it easier to scan for the right choice

* Update PowerShell version

* Exit build.ps1 if PowerShell version less than 5

* Move check, tidy up formatting

This has shown up as a hot allocation path in a few different traces.
This change amortizes the allocations by using a pooled ArrayBuilder
instead of allocating a new Stack every time.

The following benchmarks seem to support the hypothesis:

Before change:

```
BenchmarkDotNet=v0.11.1, OS=Windows 10.0.17134.590 (1803/April2018Update/Redstone4)
AMD Ryzen 7 1800X Eight-Core Processor (Max: 3.60GHz), 1 CPU, 16 logical and 8 physical cores
.NET Core SDK=2.2.103
  [Host] : .NET Core 2.1.8 (CoreCLR 4.6.27317.03, CoreFX 4.6.27317.03), 64bit RyuJIT
  Core   : .NET Core 2.1.8 (CoreCLR 4.6.27317.03, CoreFX 4.6.27317.03), 64bit RyuJIT

Job=Core  Runtime=Core  Server=True

                Method |      Mean |     Error |    StdDev |     Gen 0 | Allocated |
---------------------- |----------:|----------:|----------:|----------:|----------:|
               Parsing |  65.25 ms |  1.301 ms |  2.568 ms |         - |   6.57 MB |
 CompileMethodsAndEmit | 634.00 ms | 12.640 ms | 22.468 ms | 1000.0000 |  40.29 MB |
     SerializeMetadata | 200.52 ms |  3.656 ms |  3.241 ms | 1000.0000 |  17.39 MB |
```

After change:

```
BenchmarkDotNet=v0.11.1, OS=Windows 10.0.17134.590 (1803/April2018Update/Redstone4)
AMD Ryzen 7 1800X Eight-Core Processor (Max: 3.60GHz), 1 CPU, 16 logical and 8 physical cores
.NET Core SDK=2.2.103
  [Host] : .NET Core 2.1.8 (CoreCLR 4.6.27317.03, CoreFX 4.6.27317.03), 64bit RyuJIT
  Core   : .NET Core 2.1.8 (CoreCLR 4.6.27317.03, CoreFX 4.6.27317.03), 64bit RyuJIT

Job=Core  Runtime=Core  Server=True

                Method |      Mean |     Error |    StdDev |     Gen 0 | Allocated |
---------------------- |----------:|----------:|----------:|----------:|----------:|
               Parsing |  67.26 ms |  1.344 ms |  3.321 ms |         - |   2.59 MB |
 CompileMethodsAndEmit | 627.23 ms | 13.307 ms | 23.995 ms | 1000.0000 |  28.85 MB |
     SerializeMetadata | 206.02 ms |  4.087 ms |  5.991 ms | 1000.0000 |  17.39 MB |
```

Introduce more efficient internal representation of a sequence VirtualChars

Remove unnecessary (duplicate) WaitForAsyncOperations

Closes #31748

Move leading trivia with node when removing unused values.  Resolves #33312

Fix #33253: Smart Indent does not handle fluent sequences correctly

Rename and cleanup some members in the 'use var' feature.