[libcpp] Reimplement mkdeps data structures

[libcpp] Reimplement mkdeps data structures

https://gcc.gnu.org/ml/gcc-patches/2019-05/msg00293.html
	* include/mkdeps.h (deps_write): Add PHONY arg.
	(deps_phony_targets): Delete.
	* init.c (cpp_finish): Just call deps_write.
	* mkdeps.c (struct mkdeps): Add local vector class.  Reimplement
	vector handling.
	(munge): Munge to static buffer.
	(apply_vpath): Adjust vector handling.
	(deps_init, deps_free): Use new, delete.
	(deps_add_target): Do not munge here.  Record quoting low water mark.
	(deps_add_dep): Do not munge here.
	(deps_add_vpath): Adjust vector handling.
	(make_write_name): New.  Munge on demand here.
	(make_write_vec): New.
	(deps_phony_targets): Delete.
	(make_write): New.
	(deps_write): Forward to deps_Write.
	(deps_save, deps_restore): Adjust vector handling.

From-SVN: r270943