Replaced std::exit() with return Carbon::ErrorOr for expected errors like invalid syntax (#1120)

* Replaced std::exit() with return llvm::Expected/llvm::Error<T> for expected errors like invalid syntax.

* Use llvm::formatv() for formatting lexer error messages.
x

* Addresed merge errors.

* Fixed impl scope.

* Made ErrorBuilder::operator<< nodiscard, to catch code forgetting 'return' in 'return FATAL_COMPILATION_ERROR()'.

* FatalComplationError() -> ParseAndLexContext::RecordLexerError().
Other usages of ERROR_TOKEN in lexer.lpp were actually supposed to be END_OF_FILE.

* Update executable_semantics/syntax/parse_and_lex_context.h

Co-authored-by: Jon Meow <jperkins@google.com>

* Code review fixes.

* Update executable_semantics/syntax/parser.ypp

Co-authored-by: Jon Meow <jperkins@google.com>

* More code review fixes.

* Update executable_semantics/interpreter/type_checker.h

Co-authored-by: Jon Meow <jperkins@google.com>

* Yet more code review fixes...

* Update executable_semantics/syntax/lexer.lpp

Co-authored-by: Jon Meow <jperkins@google.com>

* code review comments

* Update executable_semantics/interpreter/interpreter.cpp

Co-authored-by: Geoff Romer <gromer@google.com>

* Apply suggestions from code review

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/syntax/lexer.lpp

Co-authored-by: Jon Meow <jperkins@google.com>

* code review

* code review

* Apply suggestions from code review

Co-authored-by: Jon Meow <jperkins@google.com>

* formatted code

* review comments

* Switched to the new ErrorOr<V> error implementation

* code review comments

* fixed comment

* restored ostream.h as #976 makes the change unnecesary

* review comments

Co-authored-by: Jon Meow <jperkins@google.com>
Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/syntax/parse_and_lex_context.cpp

@@ -6,11 +6,21 @@
 
 namespace Carbon {
 
-auto ParseAndLexContext::PrintDiagnostic(const std::string& message) -> void {
-  // TODO: Do we really want this to be fatal?  It makes the comment and the
-  // name a lie, and renders some of the other yyparse() result propagation code
-  // moot.
-  FATAL_COMPILATION_ERROR(source_loc()) << message;
+auto ParseAndLexContext::RecordSyntaxError(const std::string& message,
+                                           bool prefix_with_newline)
+    -> Parser::symbol_type {
+  // Optionally adds a newline in trace mode because trace prints an incomplete
+  // line "Reading a token: " which can prevent LIT from finding expected
+  // patterns.
+  // TODO: support formatting of `SourceLocation` instances with formatv().
+  std::string full_message;
+  llvm::raw_string_ostream(full_message)
+      << (prefix_with_newline && trace() ? "\n" : "")
+      << "COMPILATION ERROR: " << source_loc() << ": " << message;
+  error_messages_.push_back(full_message);
+
+  // TODO: use `YYerror` token once bison is upgraded to at least 3.5.
+  return Parser::make_END_OF_FILE(current_token_position);
 }
 
 }  // namespace Carbon