Rollup merge of #34495 - jseyfried:only_ident_macro_invocations, r=eddyb

Forbid type parameters and global paths in macro invocations

Fixes #28558.
This is a [breaking-change]. For example, the following would break:
```rust
macro_rules! m { () => { () } }
fn main() {
    m::<T>!(); // Type parameters are no longer allowed in macro invocations
    ::m!(); // Global paths are no longer allowed in macro invocations
}
```
Any breakage can be fixed by removing the type parameters or the leading `::` (respectively).

r? @eddyb

src/libsyntax/ext/expand.rs

@@ -202,7 +202,7 @@ fn expand_mac_invoc<T>(mac: ast::Mac, ident: Option<Ident>, attrs: Vec<ast::Attr
                                           &fld.cx.ecfg.features.unwrap());
         }
 
-        if path.segments.len() > 1 {
+        if path.segments.len() > 1 || path.global || !path.segments[0].parameters.is_empty() {
             fld.cx.span_err(path.span, "expected macro name without module separators");
             return None;
         }