* Forbid 'default' as a case constant or a pattern.

Fixes #23499

* Accept PE Operator Symbols having in parameters

* Address PR Comments

* Prefer by-val methods over in methdos in overload resolution

* More tests

* Moved check to the end of OR chain

* Address PR comments

* Operators overloading

* Address PR Comments

* Clean up

Go To Disassembly

Fix UseNullPropagation codefix doesn't recognize `x is null ? null : x.y` pattern

issue when trying to get a property access modifier of a expression-b…

Pool the Stopwatch instance used in the analyzer driver

Lazily calculate values for GetBlockActionsByAnalyzer

Use ConcurrentDictionary to avoid locks in IsGeneratedCode and HasHiddenRegions

Rebuilt xlf files to fix build break

Here is the output from my perf testing tool currently in review that
shows a significant difference in csc.exe after the change:

```
// * Detailed results *
PlaceholderBenchmarkRunner.PlaceholderMethod: Job-TGYVCW(Toolchain=Perf.ExternalProcessToolchain, LaunchCount=0, RunStrategy=Monitoring, TargetCount=25, WarmupCount=0) [Commit=HEAD^]
Runtime = ; GC =
Mean = 4.8018 s, StdErr = 0.0215 s (0.45%); N = 25, StdDev = 0.1073 s
Min = 4.6717 s, Q1 = 4.7297 s, Median = 4.7892 s, Q3 = 4.8438 s, Max = 5.1998 s
IQR = 0.1141 s, LowerFence = 4.5585 s, UpperFence = 5.0149 s
ConfidenceInterval = [4.7214 s; 4.8822 s] (CI 99.9%), Margin = 0.0804 s (1.67% of Mean)
Skewness = 1.87, Kurtosis = 7.98

PlaceholderBenchmarkRunner.PlaceholderMethod: Job-TGYVCW(Toolchain=Perf.ExternalProcessToolchain, LaunchCount=0, RunStrategy=Monitoring, TargetCount=25, WarmupCount=0) [Commit=HEAD]
Runtime = ; GC =
Mean = 3.6409 s, StdErr = 0.0650 s (1.78%); N = 25, StdDev = 0.3249 s
Min = 3.4381 s, Q1 = 3.5510 s, Median = 3.5781 s, Q3 = 3.6189 s, Max = 5.1720 s
IQR = 0.0679 s, LowerFence = 3.4491 s, UpperFence = 3.7209 s
ConfidenceInterval = [3.3975 s; 3.8842 s] (CI 99.9%), Margin = 0.2433 s (6.68% of Mean)
Skewness = 4.16, Kurtosis = 19.76

Total time: 00:07:11 (431.13 sec)

// * Summary *

BenchmarkDotNet=v0.10.9, OS=Windows 10 Redstone 1 (10.0.14393)
Processor=Intel Xeon CPU E5645 2.40GHz, ProcessorCount=24
Frequency=2337892 Hz, Resolution=427.7358 ns, Timer=TSC
.NET Core SDK=2.1.1-preview-007094
  [Host] : .NET Core 2.0.0 (Framework 4.6.00001.0), 64bit RyuJIT

Toolchain=Perf.ExternalProcessToolchain  LaunchCount=0  RunStrategy=Monitoring
TargetCount=25  WarmupCount=0

            Method | Commit |    Mean |    Error |   StdDev | Rank |
------------------ |------- |--------:|---------:|---------:|-----:|
 PlaceholderMethod |   HEAD | 3.641 s | 0.2433 s | 0.3249 s |    1 |
 PlaceholderMethod |  HEAD^ | 4.802 s | 0.0804 s | 0.1073 s |    2 |

// * Legends *
  Commit : Value of the 'Commit' parameter
  Mean   : Arithmetic mean of all measurements
  Error  : Half of 99.9% confidence interval
  StdDev : Standard deviation of all measurements
  Rank   : Relative position of current benchmark mean among all benchmarks (Arabic style)
  1 s    : 1 Second (1 sec)

```

Move to minimal MSBuild syntax 

The templates project was not being picked up by xlf previously due to
resx files being included in `<None>` items in the project file. Once we
moved to the new SDK here they were turned into resources by virtue of
the EnableDefaultEmbeddedResourceItems property. Disabling that for now
as it gets us back to the state before this change. Filed a bug to track
re-enabling it at a later time.

https://github.com/dotnet/roslyn/issues/23649

Need to wrap the property group in the test Directory.Build.props file
with a PropertyGroup.

added a way to set logging information synced to remote host and more logging for error list. (#23614)

Revert "Merge pull request #23364 from tmat/DisableBackgroundCompiler"

This reverts commit 80d79c40, reversing
changes made to 37ef0cde.

General improvements to Linux story

Fix flaky compiler server test

Optimize ArrayBuilder<T>.ToImmutableAndFree

Reduce the number of attributes that should be bound in order to determine if there are NoPia local types explicitly defined in code. (#23505)

Fixes #23179.

Add VBCSCompiler.dll to the Microsoft.NETCore.Compilers package

Perform a lightweight syntax check before getting document options

VS should be inheritting the %TEMP% value we are setting. On the off
chance it gets the original value though lets copy the props / targets
files there to see if this is the cause of the hang.

* Refactored synthesized attributes tests

* Remove rest of calls to CompilationUtils.GetSynthesizedAttributes()

* Fix EE tests

* Fix [Dynamic] tests

* Clean up

* Remove EmitMetadataTestBase.ValidateDeclSecurity(Compilation)

* Remove C# GetDefaultPEBuilder

* Fix Assembly Attributes tests

* Clean up

* Address PR Comments

* Clean up

The XML doc files for multi-targetted builds were clashing with each
other. Both were being written to the raw obj directory. Needs to be
written to the obj directory with the target framework suffix to avoid
the clash.