errors: use -Z terminal-width in JSON emitter

This commit makes the JSON emitter use `-Z terminal-width` in the
"rendered" field of the JSON output.

Signed-off-by: David Wood <david@davidtw.co>

src/test/ui/terminal-width/flag-human.rs

@@ -0,0 +1,9 @@
+// compile-flags: -Z terminal-width=20
+
+// This test checks that `-Z terminal-width` effects the human error output by restricting it to an
+// arbitrarily low value so that the effect is visible.
+
+fn main() {
+    let _: () = 42;
+    //~^ ERROR mismatched types
+}

src/test/ui/terminal-width/flag-human.stderr

@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/flag-human.rs:7:17
+   |
+LL | ..._: () = 42;
+   |       --   ^^ expected `()`, found integer
+   |       |
+   |       expected due to this
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/terminal-width/flag-json.rs

@@ -0,0 +1,9 @@
+// compile-flags: -Z terminal-width=20 --error-format=json
+
+// This test checks that `-Z terminal-width` effects the JSON error output by restricting it to an
+// arbitrarily low value so that the effect is visible.
+
+fn main() {
+    let _: () = 42;
+    //~^ ERROR mismatched types
+}

src/test/ui/terminal-width/flag-json.stderr

@@ -0,0 +1,32 @@
+{"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
+
+Erroneous code example:
+
+```compile_fail,E0308
+let x: i32 = \"I am not a number!\";
+//     ~~~   ~~~~~~~~~~~~~~~~~~~~
+//      |             |
+//      |    initializing expression;
+//      |    compiler infers type `&str`
+//      |
+//    type `i32` assigned to variable `x`
+```
+
+This error occurs when the compiler is unable to infer the concrete type of a
+variable. It can occur in several cases, the most common being a mismatch
+between two types: the type the author explicitly assigned, and the type the
+compiler inferred.
+"},"level":"error","spans":[{"file_name":"$DIR/flag-json.rs","byte_start":244,"byte_end":246,"line_start":7,"line_end":7,"column_start":17,"column_end":19,"is_primary":true,"text":[{"text":"    let _: () = 42;","highlight_start":17,"highlight_end":19}],"label":"expected `()`, found integer","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/flag-json.rs","byte_start":239,"byte_end":241,"line_start":7,"line_end":7,"column_start":12,"column_end":14,"is_primary":false,"text":[{"text":"    let _: () = 42;","highlight_start":12,"highlight_end":14}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[],"rendered":"error[E0308]: mismatched types
+  --> $DIR/flag-json.rs:7:17
+   |
+LL | ..._: () = 42;
+   |       --   ^^ expected `()`, found integer
+   |       |
+   |       expected due to this
+
+"}
+{"message":"aborting due to previous error","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to previous error
+
+"}
+{"message":"For more information about this error, try `rustc --explain E0308`.","code":null,"level":"failure-note","spans":[],"children":[],"rendered":"For more information about this error, try `rustc --explain E0308`.
+"}