[libc++][test] Improves transitive includes.

This test generates the include graph of the Standard headers of libc++ in
a CSV file. This was originally used to generate graphviz dot files. During
review it was noticed these files have all information needed to replace
the current transitive includes. Therefore the output, with the same information as the .dot file is stored in a .csv file. This removes
all the existing transitive include files.

The .cvs can be converted by a .dot file by the script in D134188.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D133127