Emit error instead of ICE when loading files with trailing '>'

Loading files with a leading '<' has been supported since Rust 1.0, so
we must keep support for that:

    touch '<leading-lt'
    echo 'include!("<leading-lt");' > leading-lt.rs
    rustc +1.0 --crate-type lib leading-lt.rs    # No error

compiler/rustc_parse/src/lib.rs

@@ -152,14 +152,28 @@ fn try_file_to_source_file(
     path: &Path,
     spanopt: Option<Span>,
 ) -> Result<Lrc<SourceFile>, Diagnostic> {
-    sess.source_map().load_file(path).map_err(|e| {
-        let msg = format!("couldn't read {}: {}", path.display(), e);
+    let mut res = if path.to_string_lossy().ends_with('>') {
+        // The file name can't end with '>' because internally in the compiler
+        // we use the file name "<...>" to represent files that are not actually
+        // real files. To avoid having to split up paths into their components,
+        // we do not bother to check for '<' however. This means that file names
+        // with just a leading '<' will be loadable.
+        let msg = format!("can't load '{}' because its file name ends with '>'", path.display());
         let mut diag = Diagnostic::new(Level::Fatal, msg);
-        if let Some(sp) = spanopt {
-            diag.set_span(sp);
-        }
-        diag
-    })
+        diag.help("remove the trailing '>' from the file name");
+        Err(diag)
+    } else {
+        sess.source_map().load_file(path).map_err(|e| {
+            let msg = format!("couldn't read {}: {}", path.display(), e);
+            Diagnostic::new(Level::Fatal, msg)
+        })
+    };
+
+    if let Err(diag) = &mut res && let Some(sp) = spanopt {
+        diag.set_span(sp);
+    }
+
+    res
 }
 
 /// Given a session and a path and an optional span (for error reporting),

tests/ui/include-macros/leading-lt.rs

@@ -0,0 +1,6 @@
+// run-pass
+// check-run-results
+
+fn main() {
+    println!(include!("silly-file-names/<leading-lt"));
+}

tests/ui/include-macros/leading-lt.run.stdout

@@ -0,0 +1 @@
+supported for backwards compatibility

tests/ui/include-macros/silly-file-names/<leading-lt

@@ -0,0 +1 @@
+"supported for backwards compatibility"

tests/ui/include-macros/trailing-gt.rs

@@ -0,0 +1,3 @@
+fn main() {
+    include!("silly-file-names/trailing-gt>"); //~ ERROR can't load '$DIR/silly-file-names/trailing-gt>' because its file name ends with '>'
+}

tests/ui/include-macros/trailing-gt.stderr

@@ -0,0 +1,11 @@
+error: can't load '$DIR/silly-file-names/trailing-gt>' because its file name ends with '>'
+  --> $DIR/trailing-gt.rs:2:5
+   |
+LL |     include!("silly-file-names/trailing-gt>");
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: remove the trailing '>' from the file name
+   = note: this error originates in the macro `include` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+