Audit uses of apply_mark in built-in macros

Replace them with equivalents of `Span::{def_site,call_site}` from proc macro API.
The new API is much less error prone and doesn't rely on macros having default transparency.

src/libsyntax_ext/global_allocator.rs

@@ -3,7 +3,6 @@ use syntax::ast::{self, Arg, Attribute, Expr, FnHeader, Generics, Ident};
 use syntax::attr::check_builtin_macro_attribute;
 use syntax::ext::allocator::{AllocatorKind, AllocatorMethod, AllocatorTy, ALLOCATOR_METHODS};
 use syntax::ext::base::{Annotatable, ExtCtxt};
-use syntax::ext::hygiene::SyntaxContext;
 use syntax::ptr::P;
 use syntax::symbol::{kw, sym, Symbol};
 use syntax_pos::Span;
@@ -29,7 +28,7 @@ pub fn expand(
     };
 
     // Generate a bunch of new items using the AllocFnFactory
-    let span = item.span.with_ctxt(SyntaxContext::root().apply_mark(ecx.current_expansion.id));
+    let span = ecx.with_legacy_ctxt(item.span);
     let f = AllocFnFactory {
         span,
         kind: AllocatorKind::Global,