[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/debug.cpp b/src/debug.cpp
index 15d8fdb..14a75b1 100644
--- a/src/debug.cpp
+++ b/src/debug.cpp
@@ -6,18 +6,19 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "__config"
-#include "__debug"
-#include "functional"
-#include "algorithm"
-#include "string"
-#include "cstdio"
-#include "__hash_table"
+#include <__config>
+#include <__debug>
+#include <__hash_table>
+#include <algorithm>
+#include <cstdio>
+#include <functional>
+#include <string>
+
 #ifndef _LIBCPP_HAS_NO_THREADS
-#include "mutex"
-#if defined(__ELF__) && defined(_LIBCPP_LINK_PTHREAD_LIB)
-#pragma comment(lib, "pthread")
-#endif
+#  include <mutex>
+#  if defined(__ELF__) && defined(_LIBCPP_LINK_PTHREAD_LIB)
+#    pragma comment(lib, "pthread")
+#  endif
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD