Switch -ftrack-macro-expansion=2 on by default.

This switches the compiler to -ftrack-macro-expansion=2 by default.

Tested and bootstrapped on x86_64-unknown-linux-gnu against trunk.

libcpp/

	* init.c (cpp_create_reader): Switch -ftrack-macro-expansion=2 on
	by default.  Add comments.

gcc/docs/

	* cppopts.texi: Adjust for enabling -ftrack-macro-expansion=2 by
	default.

From-SVN: r186977