[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
diff --git a/libcxx/src/regex.cpp b/libcxx/src/regex.cpp
index 16ad8f0..e53d324 100644
--- a/libcxx/src/regex.cpp
+++ b/libcxx/src/regex.cpp
@@ -6,9 +6,9 @@
//
//===----------------------------------------------------------------------===//
-#include "regex"
-#include "algorithm"
-#include "iterator"
+#include <algorithm>
+#include <iterator>
+#include <regex>
_LIBCPP_BEGIN_NAMESPACE_STD
