Rotate ChangeLog files - part 1 - add ChangeLog-2020.

2021-01-01  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* ChangeLog-2020: Rotate ChangeLog.  New file.
gcc/ada/
	* ChangeLog-2020: Rotate ChangeLog.  New file.
gcc/cp/
	* ChangeLog-2020: Rotate ChangeLog.  New file.
gcc/d/
	* ChangeLog-2020: Rotate ChangeLog.  New file.
gcc/fortran/
	* ChangeLog-2020: Rotate ChangeLog.  New file.
gcc/testsuite/
	* ChangeLog-2020: Rotate ChangeLog.  New file.
libgfortran/
	* ChangeLog-2020: Rotate ChangeLog.  New file.
libstdc++-v3/
	* ChangeLog-2020: Rotate ChangeLog.  New file.