Allow CSharpSyntaxGenerator to be used as a source generator

This takes a fairly hacky approach to updating the code by converting
the CSharpSyntaxGenerator project to build twice: once as a
netcoreapp3.1 console app like it always did, and now also as a
netstandard2.0 library that is a source generator. The source generator
one only supports building the .cs outputs that are consumed as regular
APIs, and ignores the test or grammar outputs for now.

There is also no handling for cancellation yet which is not good for
performance.

Compilers.sln

@@ -301,10 +301,10 @@ Global
 		{288089C5-8721-458E-BE3E-78990DAB5E2E}.Debug|Any CPU.Build.0 = Debug|x64
 		{288089C5-8721-458E-BE3E-78990DAB5E2E}.Release|Any CPU.ActiveCfg = Release|x64
 		{288089C5-8721-458E-BE3E-78990DAB5E2E}.Release|Any CPU.Build.0 = Release|x64
-		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Debug|Any CPU.ActiveCfg = Debug|x64
-		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Debug|Any CPU.Build.0 = Debug|x64
-		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Release|Any CPU.ActiveCfg = Release|x64
-		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Release|Any CPU.Build.0 = Release|x64
+		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Release|Any CPU.Build.0 = Release|Any CPU
 		{D0A79850-B32A-45E5-9FD5-D43CB345867A}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{D0A79850-B32A-45E5-9FD5-D43CB345867A}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{D0A79850-B32A-45E5-9FD5-D43CB345867A}.Release|Any CPU.ActiveCfg = Release|Any CPU

Roslyn.sln

@@ -662,10 +662,10 @@ Global
 		{288089C5-8721-458E-BE3E-78990DAB5E2E}.Debug|Any CPU.Build.0 = Debug|x64
 		{288089C5-8721-458E-BE3E-78990DAB5E2E}.Release|Any CPU.ActiveCfg = Release|x64
 		{288089C5-8721-458E-BE3E-78990DAB5E2E}.Release|Any CPU.Build.0 = Release|x64
-		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Debug|Any CPU.ActiveCfg = Debug|x64
-		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Debug|Any CPU.Build.0 = Debug|x64
-		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Release|Any CPU.ActiveCfg = Release|x64
-		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Release|Any CPU.Build.0 = Release|x64
+		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{288089C5-8721-458E-BE3E-78990DAB5E2D}.Release|Any CPU.Build.0 = Release|Any CPU
 		{6AA96934-D6B7-4CC8-990D-DB6B9DD56E34}.Debug|Any CPU.ActiveCfg = Debug|x64
 		{6AA96934-D6B7-4CC8-990D-DB6B9DD56E34}.Debug|Any CPU.Build.0 = Debug|x64
 		{6AA96934-D6B7-4CC8-990D-DB6B9DD56E34}.Release|Any CPU.ActiveCfg = Release|x64

eng/generate-compiler-code.ps1

@@ -11,7 +11,7 @@ $ErrorActionPreference="Stop"
 function Run-Tool($projectFilePath, $toolArgs) {
   $toolName = Split-Path -leaf $projectFilePath
   Write-Host "Running $toolName $toolArgs"
-  Exec-Console $dotnet "run -p $projectFilePath $toolArgs"
+  Exec-Console $dotnet "run -p $projectFilePath --framework netcoreapp3.1 $toolArgs"
 }
 
 function Run-LanguageCore($language, $languageSuffix, $languageDir, $syntaxProject, $errorFactsProject, $generatedDir, $generatedTestDir) {
@@ -25,7 +25,11 @@ function Run-LanguageCore($language, $languageSuffix, $languageDir, $syntaxProje
 
   Create-Directory $generatedDir
   Create-Directory $generatedTestDir
-  Run-Tool $syntaxProject "`"$syntaxFilePath`" `"$generatedDir`""
+
+  # The C# syntax is now generated by a source generator
+  if ($language -ne "CSharp") {
+	  Run-Tool $syntaxProject "`"$syntaxFilePath`" `"$generatedDir`""
+  }
   Run-Tool $syntaxProject "`"$syntaxFilePath`" `"$generatedDir`" /grammar"
   Run-Tool $syntaxProject "`"$syntaxFilePath`" `"$syntaxTestFilePath`" /test"
   Run-Tool $boundTreeGenProject "$language `"$boundFilePath`" `"$boundGeneratedFilePath`""

eng/targets/Imports.targets

@@ -171,7 +171,7 @@
       Disable analyzers via an MSBuild property settable on the command line.
     -->
     <ItemGroup>
-      <Analyzer Remove="@(Analyzer)" />
+      <Analyzer Remove="@(Analyzer)" Condition="'%(FileName)' != 'CSharpSyntaxGenerator'" />
     </ItemGroup>
   </Target>