[Diagnostics] Introduce "Educational Notes" for diagnostics

Educational notes are small pieces of documentation which explain a concept
relevant to some diagnostic message. If -enable-descriptive-diagnostics is
passed, they will be printed after a diagnostic message if available.

Educational notes can be found at /usr/share/doc/diagnostics in a
toolchain, and are associated with specific compiler diagnostics in
EducationalNotes.def.

lib/Frontend/CompilerInvocation.cpp

@@ -44,6 +44,13 @@ void CompilerInvocation::setMainExecutablePath(StringRef Path) {
   llvm::sys::path::remove_filename(LibPath); // Remove /bin
   llvm::sys::path::append(LibPath, "lib", "swift");
   setRuntimeResourcePath(LibPath.str());
+
+  llvm::SmallString<128> DiagnosticDocsPath(Path);
+  llvm::sys::path::remove_filename(DiagnosticDocsPath); // Remove /swift
+  llvm::sys::path::remove_filename(DiagnosticDocsPath); // Remove /bin
+  llvm::sys::path::append(DiagnosticDocsPath, "share", "doc", "swift",
+                          "diagnostics");
+  DiagnosticOpts.DiagnosticDocumentationPath = DiagnosticDocsPath.str();
 }
 
 /// If we haven't explicitly passed -prebuilt-module-cache-path, set it to
@@ -682,7 +689,9 @@ static bool ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
   Opts.PrintDiagnosticNames |= Args.hasArg(OPT_debug_diagnostic_names);
   Opts.EnableDescriptiveDiagnostics |=
       Args.hasArg(OPT_enable_descriptive_diagnostics);
-
+  if (Arg *A = Args.getLastArg(OPT_diagnostic_documentation_path)) {
+    Opts.DiagnosticDocumentationPath = A->getValue();
+  }
   assert(!(Opts.WarningsAsErrors && Opts.SuppressWarnings) &&
          "conflicting arguments; should have been caught by driver");