gccrs: resolve: Remove ProcMacroInvocable interface

gccrs: resolve: Remove ProcMacroInvocable interface

Since all identifiers in attributes are converted to SimplePath, this
common interface is no longer required.

gcc/rust/ChangeLog:

	* ast/rust-ast.h (class Identifier): Remove interface
	inheritance.
	(class SimplePath): Likewise.
	* expand/rust-expand-visitor.cc (get_traits_to_derive): Change
	return type.
	(derive_item): Update according to get_traits_to_derive return
	type.
	(expand_item_attribute): Likewise.
	(ExpandVisitor::expand_inner_stmts): Likewise.
	* expand/rust-macro-expand.h (struct MacroExpander): Likewise.
	* util/rust-hir-map.cc (Mappings::insert_derive_proc_macro_invocation):
	Change input type to SimplePath.
	(Mappings::lookup_derive_proc_macro_invocation): Likewise.
	(Mappings::insert_attribute_proc_macro_invocation): Likewise.
	(Mappings::lookup_attribute_proc_macro_invocation): Likewise.
	* util/rust-hir-map.h: Likewise with function prototypes.
	* util/rust-proc-macro-invocation.h: Removed.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>