[libc++] Map forward declaration headers for iostreams to <iosfwd> (#83327)

This seems more appropriate than mapping them to the headers that
contain actual definitions.

Author: ldionne

libcxx/include/libcxx.imp

@@ -425,17 +425,17 @@
   { include: [ "<__fwd/bit_reference.h>", "private", "<bitset>", "public" ] },
   { include: [ "<__fwd/bit_reference.h>", "private", "<vector>", "public" ] },
   { include: [ "<__fwd/complex.h>", "private", "<complex>", "public" ] },
-  { include: [ "<__fwd/fstream.h>", "private", "<fstream>", "public" ] },
+  { include: [ "<__fwd/fstream.h>", "private", "<iosfwd>", "public" ] },
   { include: [ "<__fwd/hash.h>", "private", "<functional>", "public" ] },
-  { include: [ "<__fwd/ios.h>", "private", "<ios>", "public" ] },
-  { include: [ "<__fwd/istream.h>", "private", "<istream>", "public" ] },
+  { include: [ "<__fwd/ios.h>", "private", "<iosfwd>", "public" ] },
+  { include: [ "<__fwd/istream.h>", "private", "<iosfwd>", "public" ] },
   { include: [ "<__fwd/mdspan.h>", "private", "<mdspan>", "public" ] },
   { include: [ "<__fwd/memory_resource.h>", "private", "<memory_resource>", "public" ] },
-  { include: [ "<__fwd/ostream.h>", "private", "<ostream>", "public" ] },
+  { include: [ "<__fwd/ostream.h>", "private", "<iosfwd>", "public" ] },
   { include: [ "<__fwd/pair.h>", "private", "<utility>", "public" ] },
   { include: [ "<__fwd/span.h>", "private", "<span>", "public" ] },
-  { include: [ "<__fwd/sstream.h>", "private", "<sstream>", "public" ] },
-  { include: [ "<__fwd/streambuf.h>", "private", "<streambuf>", "public" ] },
+  { include: [ "<__fwd/sstream.h>", "private", "<iosfwd>", "public" ] },
+  { include: [ "<__fwd/streambuf.h>", "private", "<iosfwd>", "public" ] },
   { include: [ "<__fwd/string.h>", "private", "<string>", "public" ] },
   { include: [ "<__fwd/string_view.h>", "private", "<string_view>", "public" ] },
   { include: [ "<__fwd/subrange.h>", "private", "<ranges>", "public" ] },

libcxx/utils/generate_iwyu_mapping.py

@@ -40,6 +40,8 @@ def IWYU_mapping(header: str) -> typing.Optional[typing.List[str]]:
         return ["utility"]
     elif header == "__fwd/subrange.h":
         return ["ranges"]
+    elif re.match("__fwd/(fstream|ios|istream|ostream|sstream|streambuf)[.]h", header):
+        return ["iosfwd"]
     # Handle remaining forward declaration headers
     elif re.match("__fwd/(.+)[.]h", header):
         return [re.match("__fwd/(.+)[.]h", header).group(1)]