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.h

@@ -13,20 +13,17 @@
 
 namespace Carbon {
 
-// This is the code given us by Bison, for now.
-using SyntaxErrorCode = int;
-
 // Returns the AST representing the contents of the named file, or an error code
 // if parsing fails. Allocations go into the provided arena.
 auto Parse(Nonnull<Arena*> arena, std::string_view input_file_name, bool trace)
-    -> std::variant<Carbon::AST, SyntaxErrorCode>;
+    -> ErrorOr<Carbon::AST>;
 
 // Equivalent to `Parse`, but parses the contents of `file_contents`.
 // `input_file_name` is used only for reporting source locations, and does
 // not need to name a real file.
 auto ParseFromString(Nonnull<Arena*> arena, std::string_view input_file_name,
                      std::string_view file_contents, bool trace)
-    -> std::variant<Carbon::AST, SyntaxErrorCode>;
+    -> ErrorOr<Carbon::AST>;
 
 }  // namespace Carbon