Switch from llvm::errs()+exit() to FATAL_USER_ERROR() stream-based handling. (#670)

This is intended to:

- Standardize use of line_num (and highlight where no line_num is available).
- Standardize indications of runtime/compilation errors.
- Make it incrementally easier to write new errors.

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

executable_semantics/testdata/experimental_continuation9.golden

@@ -1,2 +1,2 @@
-12: undefined behavior: access to dead value 1
+RUNTIME ERROR: 12: : undefined behavior: access to dead value 1
 EXIT CODE: 255

executable_semantics/testdata/fun6_fail_type.golden

@@ -1,2 +1,2 @@
-10: compilation error, tuples of different length
+COMPILATION ERROR: 10: tuples of different length
 EXIT CODE: 255

executable_semantics/testdata/generic_function_fail1.golden

@@ -1,4 +1,4 @@
-ERROR: 10: type error in argument deduction
+COMPILATION ERROR: 10: type error in argument deduction
 expected: Int
 actual: Bool
 EXIT CODE: 255

executable_semantics/testdata/generic_function_fail3.golden

@@ -1,4 +1,4 @@
-ERROR: 6: type error in addition(1)
+COMPILATION ERROR: 6: type error in addition(1)
 expected: Int
 actual: T
 EXIT CODE: 255

executable_semantics/testdata/global_variable3.golden

@@ -1,4 +1,4 @@
-ERROR: 7: type error in initializer of variable
+COMPILATION ERROR: 7: type error in initializer of variable
 expected: Int
 actual: Bool
 EXIT CODE: 255

executable_semantics/testdata/global_variable5.golden

@@ -1,4 +1,4 @@
-ERROR: 10: type error in return
+COMPILATION ERROR: 10: type error in return
 expected: ()
 actual: Int
 EXIT CODE: 255

executable_semantics/testdata/global_variable8.golden

@@ -1,2 +1,2 @@
-8: could not find `y`
+RUNTIME ERROR: 8: : could not find `y`
 EXIT CODE: 255

executable_semantics/testdata/pattern_variable_fail.golden

@@ -1,2 +1,2 @@
-7: compilation error, pattern variables are only allowed in pattern context
+COMPILATION ERROR: 7: pattern variables are only allowed in pattern context
 EXIT CODE: 255

executable_semantics/testdata/struct_field_access_mismatch.carbon

@@ -0,0 +1,12 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+struct Point {
+  var x: Int;
+  var y: Int;
+}
+
+fn main() -> Int {
+  return Point(.x = 1, .y = 2).z - 1;
+}

executable_semantics/testdata/struct_field_access_mismatch.golden

@@ -0,0 +1,2 @@
+COMPILATION ERROR: 11: struct Point does not have a field named z
+EXIT CODE: 255

executable_semantics/testdata/struct_field_mismatch.carbon

@@ -0,0 +1,12 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+struct Point {
+  var x: Int;
+  var y: Int;
+}
+
+fn main() -> Int {
+  return Point(.x = 1, .z = 2).x - 1;
+}

executable_semantics/testdata/struct_field_mismatch.golden

@@ -0,0 +1,2 @@
+COMPILATION ERROR: 11: field names do not match, expected y but got z
+EXIT CODE: 255

executable_semantics/testdata/struct_field_missing.carbon

@@ -0,0 +1,12 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+struct Point {
+  var x: Int;
+  var y: Int;
+}
+
+fn main() -> Int {
+  return Point(.x = 1).x - 1;
+}

executable_semantics/testdata/struct_field_missing.golden

@@ -0,0 +1,2 @@
+COMPILATION ERROR: 11: tuples of different length
+EXIT CODE: 255

executable_semantics/testdata/tuple5.golden

@@ -1,4 +1,4 @@
-ERROR: 8: type error in pattern variable
+COMPILATION ERROR: 8: type error in pattern variable
 expected: (x = Int, y = Int)
 actual: (y = Int, x = Int)
 EXIT CODE: 255

executable_semantics/testdata/tuple_equality3.golden

@@ -1,4 +1,4 @@
-ERROR: 8: type error in ==
+COMPILATION ERROR: 8: type error in ==
 expected: (0 = Int, 1 = Int)
 actual: (0 = Int)
 EXIT CODE: 255