Emit LLVM optimization remarks when enabled with -Cremark

The default diagnostic handler considers all remarks to be disabled by
default unless configured otherwise through LLVM internal flags:
`-pass-remarks`, `-pass-remarks-missed`, and `-pass-remarks-analysis`.
This behaviour makes `-Cremark` ineffective on its own.

Fix this by configuring a custom diagnostic handler that enables
optimization remarks based on the value of `-Cremark` option. With
`-Cremark=all` enabling all remarks.

src/test/ui/optimization-remark.rs

@@ -0,0 +1,19 @@
+// build-pass
+// ignore-pass
+// no-system-llvm
+// revisions: all inline
+//          compile-flags: --crate-type=lib -Cdebuginfo=1 -Copt-level=2
+// [all]    compile-flags: -Cremark=all
+// [inline] compile-flags: -Cremark=inline
+// error-pattern: inline: f not inlined into g
+// dont-check-compiler-stderr
+
+#[no_mangle]
+#[inline(never)]
+pub fn f() {
+}
+
+#[no_mangle]
+pub fn g() {
+    f();
+}