Replace usage of ResumeTy in async lowering with Context

Replaces using `ResumeTy` / `get_context` in favor of using `&'static mut Context<'_>`.

Usage of the `'static` lifetime here is technically "cheating", and replaces
the raw pointer in `ResumeTy` and the `get_context` fn that pulls the
correct lifetimes out of thin air.

compiler/rustc_span/src/symbol.rs

@@ -165,6 +165,7 @@ symbols! {
         Capture,
         Center,
         Clone,
+        Context,
         Continue,
         Copy,
         Count,
@@ -264,7 +265,6 @@ symbols! {
         Relaxed,
         Release,
         Result,
-        ResumeTy,
         Return,
         Right,
         Rust,
@@ -754,7 +754,6 @@ symbols! {
         generic_associated_types_extended,
         generic_const_exprs,
         generic_param_attrs,
-        get_context,
         global_allocator,
         global_asm,
         globs,