[libc++/abi] Replace uses of _NOEXCEPT in src/ by noexcept

We always build the libraries in a Standard mode that supports noexcept,
so there's no need to use the _NOEXCEPT macro.

Differential Revision: https://reviews.llvm.org/D97700

GitOrigin-RevId: 5601305fb30bf585c397e62d2d3a8a712e47d70b
diff --git a/src/future.cpp b/src/future.cpp
index 58d0b4c..4c59f89 100644
--- a/src/future.cpp
+++ b/src/future.cpp
@@ -19,12 +19,12 @@
     : public __do_message
 {
 public:
-    virtual const char* name() const _NOEXCEPT;
+    virtual const char* name() const noexcept;
     virtual string message(int ev) const;
 };
 
 const char*
-__future_error_category::name() const _NOEXCEPT
+__future_error_category::name() const noexcept
 {
     return "future";
 }
@@ -65,7 +65,7 @@
 #endif
 
 const error_category&
-future_category() _NOEXCEPT
+future_category() noexcept
 {
     static __future_error_category __f;
     return __f;
@@ -77,12 +77,12 @@
 {
 }
 
-future_error::~future_error() _NOEXCEPT
+future_error::~future_error() noexcept
 {
 }
 
 void
-__assoc_sub_state::__on_zero_shared() _NOEXCEPT
+__assoc_sub_state::__on_zero_shared() noexcept
 {
     delete this;
 }