[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/filesystem/operations.cpp b/src/filesystem/operations.cpp
index 244cc98..57e2539 100644
--- a/src/filesystem/operations.cpp
+++ b/src/filesystem/operations.cpp
@@ -7,14 +7,14 @@
 //===----------------------------------------------------------------------===//
 
 #include <__utility/unreachable.h>
-#include "filesystem"
-#include "array"
-#include "iterator"
-#include "string_view"
-#include "type_traits"
-#include "vector"
-#include "cstdlib"
-#include "climits"
+#include <array>
+#include <climits>
+#include <cstdlib>
+#include <filesystem>
+#include <iterator>
+#include <string_view>
+#include <type_traits>
+#include <vector>
 
 #include "filesystem_common.h"
 
@@ -40,7 +40,7 @@
 # include <copyfile.h>
 # define _LIBCPP_FILESYSTEM_USE_COPYFILE
 #else
-# include "fstream"
+# include <fstream>
 # define _LIBCPP_FILESYSTEM_USE_FSTREAM
 #endif