Auto merge of #131284 - dingxiangfei2009:rename-smart-ptr-to-coerce-referent, r=compiler-errors

Rename macro `SmartPointer` to `CoercePointee`

As per resolution #129104 we will rename the macro to better reflect the technical specification of the feature and clarify the communication.

- `SmartPointer` is renamed to `CoerceReferent`
- `#[pointee]` attribute is renamed to `#[referent]`
- `#![feature(derive_smart_pointer)]` gate is renamed to `#![feature(derive_coerce_referent)]`.
- Any mention of `SmartPointer` in the file names are renamed accordingly.

r? `@compiler-errors`

cc `@nikomatsakis` `@Darksonn`

compiler/rustc_span/src/symbol.rs

@@ -174,6 +174,7 @@ symbols! {
         Center,
         Cleanup,
         Clone,
+        CoercePointee,
         CoerceUnsized,
         Command,
         ConstParamTy,
@@ -307,7 +308,6 @@ symbols! {
         Sized,
         SliceIndex,
         SliceIter,
-        SmartPointer,
         Some,
         SpanCtxt,
         String,
@@ -732,6 +732,7 @@ symbols! {
         deref_pure,
         deref_target,
         derive,
+        derive_coerce_pointee,
         derive_const,
         derive_default_enum,
         derive_smart_pointer,