Migrate remaining exits to FATAL_*_ERROR calls (#704)

Adds FATAL for things that are most likely programming errors in executable_semantics.

executable_semantics/syntax/lexer.lpp

@@ -6,7 +6,6 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 %{
 #include <cstdlib>
-#include <iostream>
 
 #include "common/check.h"
 #include "executable_semantics/common/tracing_flag.h"
@@ -215,11 +214,10 @@ operator and its operand, leading to three more cases:
   if (Carbon::tracing_output) {
     // Print a newline because tracing prints an incomplete line
     // "Reading a token: ".
-    std::cerr << std::endl;
+    llvm::errs() << "\n";
   }
-  std::cerr << context.current_token_position << ": invalid character '\\x"
-            << llvm::toHex(llvm::StringRef(yytext, 1)) << "' in source file." << std::endl;
-  std::exit(1);
+  FATAL_COMPILATION_ERROR(yylineno) << "invalid character '\\x"
+            << llvm::toHex(llvm::StringRef(yytext, 1)) << "' in source file.";
 }
 
 <<EOF>>    {

executable_semantics/syntax/paren_contents.h

@@ -71,7 +71,7 @@ auto ParenContents<Term>::TupleElements(int line_num) const
       result.push_back(TupleElement(*element.name, element.term));
     } else {
       if (seen_named_member) {
-        FATAL_USER_ERROR(line_num)
+        FATAL_PROGRAM_ERROR(line_num)
             << "positional members must come before named members";
       }
       result.push_back(TupleElement(std::to_string(i), element.term));

executable_semantics/syntax/parse.cpp

@@ -4,8 +4,6 @@
 
 #include "executable_semantics/syntax/parse.h"
 
-#include <iostream>
-
 #include "common/check.h"
 #include "executable_semantics/common/error.h"
 #include "executable_semantics/common/tracing_flag.h"
@@ -23,8 +21,8 @@ auto parse(const std::string& input_file_name)
     -> std::variant<AST, SyntaxErrorCode> {
   yyin = fopen(input_file_name.c_str(), "r");
   if (yyin == nullptr) {
-    FATAL_USER_ERROR_NO_LINE() << "Error opening '" << input_file_name
-                               << "': " << std::strerror(errno);
+    FATAL_PROGRAM_ERROR_NO_LINE() << "Error opening '" << input_file_name
+                                  << "': " << std::strerror(errno);
   }
 
   std::optional<AST> parsed_input = std::nullopt;

executable_semantics/syntax/parse_and_lex_context.cpp

@@ -4,9 +4,6 @@
 
 #include "executable_semantics/syntax/parse_and_lex_context.h"
 
-#include <cstring>
-#include <iostream>
-
 // Writes a syntax error diagnostic, containing message, for the input file at
 // the given line, to standard error.
 auto Carbon::ParseAndLexContext::PrintDiagnostic(const std::string& message,
@@ -14,7 +11,5 @@ auto Carbon::ParseAndLexContext::PrintDiagnostic(const std::string& message,
   // 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.
-  std::cerr << input_file_name << ":" << line_num << ": " << message
-            << std::endl;
-  exit(-1);
+  FATAL_COMPILATION_ERROR(line_num) << message;
 }

executable_semantics/syntax/parser.ypp

@@ -51,7 +51,6 @@
 #include <cstdarg>
 #include <cstdio>
 #include <cstdlib>
-#include <iostream>
 #include <list>
 #include <vector>