Compiler Support for AnalyzerConfig

AnalyzerConfig is an EditorConfig-superset (https://editorconfig.org/) file format recognized by the Roslyn command line compiler. The options specified in analyzer config files are recognized by the compiler in two ways: option keys following the pattern dotnet_diagnostic.<diagnostic-id>.severity = <value> are parsed and interpreted by the compiler to configure the severity of compiler diagnostics. <diagnostic-id> represents the diagnostic ID matched by the compiler, case-insensitively, to be configured. <value> must be the name of a member of the ReportDiagnostic enum, also case-insensitive. These settings are then applied on a per-SyntaxTree to each of the files whose path matched the AnalyzerConfig name specification in the compilation.

Any properties which do not have the aforementioned pattern are considered analyzer options and are placed in a PerTreeOptionsProvider on the AnalyzerOptions type for use by analyzers.

AnalyzerConfig files can be passed to the command-line compiler through the /analyzerconfig:<file-path> parameter.