[libc++] Include the full set of libc++ transitive includes in the CSV files (#107911)

When we introduced the machinery for transitive includes validation, at
some point we stopped including the full set of transitive includes in
the CSV files and instead only tracked the set of public headers
included *directly* by a top-level header.

The reason for doing that was so that the CSV files containing
"transitive" includes could be used to draw the dependency graph of
libc++ headers. However, the downside was that it made the contents of
the CSV files much harder to interpret.

In particular, many changes that modify the CSV files do not in fact
modify the effective set of transitive includes, which is confusing.
This patch goes back to storing the full set of transitive includes in
the CSV files and removes the ability to graph the libc++ includes
directly from those CSV files, which we never actually used.

Author: ldionne

libcxx/test/libcxx/transitive_includes/cxx03.csv

@@ -62,12 +62,12 @@ def is_libcxx_header(header: str) -> bool:
 
 def parse_file(file: pathlib.Path) -> List[str]:
     """
-    Parse a file containing --trace-include output to generate a list of the top-level C++ includes
+    Parse a file containing --trace-includes output to generate a list of the top-level C++ includes
     contained in it.
 
-    This effectively generates the dependency graph of C++ Standard Library headers of the header
-    whose --trace-include it is. In order to get the expected result of --trace-include, the
-    -fshow-skipped-includes flag also needs to be passed.
+    This effectively generates the list of public libc++ headers of the header whose --trace-includes it is.
+    In order to get the expected result of --trace-includes, the -fshow-skipped-includes flag also needs to
+    be passed.
     """
     result = list()
     with file.open(encoding="utf-8") as f:
@@ -87,10 +87,6 @@ def parse_file(file: pathlib.Path) -> List[str]:
                 result.append(header)
                 continue
 
-            # Skip libc++ headers included transitively.
-            if header.level > level:
-                continue
-
             # Detail headers are transparent too: we attribute all includes of public libc++
             # headers under a detail header to the last public libc++ header that included it.
             if header.name.startswith("__") or re.search(r"(/|\\\\)__", header.name):