Return ReifyShim from Instance::resolve_for_fn_ptr when track_caller present.

This ICEs in MIR currently, which I think is to be expected since none of the MIR plumbing is set up. I added a test which confirms that the shim is being used for reifying a track_caller function.

src/librustc/ty/instance.rs

@@ -1,4 +1,4 @@
-// use crate::hir::CodegenFnAttrFlags;
+use crate::hir::CodegenFnAttrFlags;
 use crate::hir::Unsafety;
 use crate::hir::def::Namespace;
 use crate::hir::def_id::DefId;
@@ -305,7 +305,7 @@ impl<'tcx> Instance<'tcx> {
         substs: SubstsRef<'tcx>,
     ) -> Option<Instance<'tcx>> {
         debug!("resolve(def_id={:?}, substs={:?})", def_id, substs);
-        if false {
+        if tcx.codegen_fn_attrs(def_id).flags.contains(CodegenFnAttrFlags::TRACK_CALLER) {
             debug!(" => fn pointer created for function with #[track_caller]");
             Some(Instance {
                 def: InstanceDef::ReifyShim(def_id),

src/test/ui/rfc-2091-track-caller/taking-fn-pointer.rs

@@ -0,0 +1,16 @@
+// failure-status: 101
+// normalize-stderr-test "note: rustc 1.* running on .*" -> "note: rustc VERSION running on TARGET"
+// normalize-stderr-test "note: compiler flags: .*" -> "note: compiler flags: FLAGS"
+
+#![feature(track_caller)] //~ WARN the feature `track_caller` is incomplete
+
+#[track_caller]
+fn f() {}
+
+fn call_it(x: fn()) {
+    x();
+}
+
+fn main() {
+    call_it(f);
+}

src/test/ui/rfc-2091-track-caller/taking-fn-pointer.stderr

@@ -0,0 +1,21 @@
+warning: the feature `track_caller` is incomplete and may cause the compiler to crash
+  --> $DIR/taking-fn-pointer.rs:5:12
+   |
+LL | #![feature(track_caller)]
+   |            ^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+
+thread 'rustc' panicked at 'index out of bounds: the len is 1 but the index is 1', $SRC_DIR/libcore/slice/mod.rs:LL:COL
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
+
+error: internal compiler error: unexpected panic
+
+note: the compiler unexpectedly panicked. this is a bug.
+
+note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
+
+note: rustc VERSION running on TARGET
+
+note: compiler flags: FLAGS
+