PR 12480
	* c-pch.c (pch_init): Improve error message when precompiled
	header can't be written.

	PR 12606
	* c-pch.c (pch_init): Make a PCH file appear invalid while it's
	being written.
	(c_common_write_pch): Make it valid once it's done.

From-SVN: r74754