[libc++] Replace `#include ""` with `<>` in libcxx/src/. NFCI.
Our best guess is that the two syntaxes should have exactly equivalent
effects, so, let's be consistent with what we do in libcxx/include/.
I've left `#include "include/x.h"` and `#include "../y.h"` alone
because I'm less sure that they're interchangeable, and they aren't
inconsistent with libcxx/include/ because libcxx/include/ never
does that kind of thing.
Also, use the `_LIBCPP_PUSH_MACROS/POP_MACROS` dance for `<__undef_macros>`,
even though it's technically unnecessary in a standalone .cpp file,
just so we have consistently one way to do it.
Differential Revision: https://reviews.llvm.org/D119561
NOKEYCHECK=True
GitOrigin-RevId: bbb0f2c759a16f2fb4e1d74795b790cf0c170620
diff --git a/src/ios.instantiations.cpp b/src/ios.instantiations.cpp
index e1189d0..2ff7057 100644
--- a/src/ios.instantiations.cpp
+++ b/src/ios.instantiations.cpp
@@ -6,14 +6,13 @@
//
//===----------------------------------------------------------------------===//
-#include "__config"
-#include "fstream"
-#include "ios"
-#include "istream"
-#include "ostream"
-#include "sstream"
-#include "streambuf"
-
+#include <__config>
+#include <fstream>
+#include <ios>
+#include <istream>
+#include <ostream>
+#include <sstream>
+#include <streambuf>
_LIBCPP_BEGIN_NAMESPACE_STD