[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/ios.cpp b/src/ios.cpp
index 3a92964..7e4d061 100644
--- a/src/ios.cpp
+++ b/src/ios.cpp
@@ -27,12 +27,12 @@
     : public __do_message
 {
 public:
-    virtual const char* name() const _NOEXCEPT;
+    virtual const char* name() const noexcept;
     virtual string message(int ev) const;
 };
 
 const char*
-__iostream_category::name() const _NOEXCEPT
+__iostream_category::name() const noexcept
 {
     return "iostream";
 }
@@ -50,7 +50,7 @@
 }
 
 const error_category&
-iostream_category() _NOEXCEPT
+iostream_category() noexcept
 {
     static __iostream_category s;
     return s;
@@ -387,7 +387,7 @@
 }
 
 void
-ios_base::swap(ios_base& rhs) _NOEXCEPT
+ios_base::swap(ios_base& rhs) noexcept
 {
     _VSTD::swap(__fmtflags_, rhs.__fmtflags_);
     _VSTD::swap(__precision_, rhs.__precision_);