fix to re-enable PCH

[Brendan Kehoe <brendan at zen dot org>]

This change fixes the compiler's ability to use the correct precompiled header 
out of a directory, which stopped working with recent changes to the PCH 
code.

We now must reset the PCH state of the _cpp_file object that's being used to 
try each entry in the directory "bits/stdc++.h.gch" for the use of
	-include bits/stdc++.h
on the command line.

The pch_open_file() while loop's first attempt, on "/some/dir/.", will fail 
and thus cause file->pch to be set to 2; without this change, all subsequent 
tries, including "/some/dir/O0g", will not be properly visited by 
validate_pch because it'll see the flag set to 2, meaning the file (even 
though it was concerning a previous one) is not and never will be a valid 
precompiled header.

Hope this helps,
B

-- 
Brendan Kehoe                                               brendan@zen.org

http://www.zen.org/~brendan/

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