Patch PING^2: fix --enable-mapped-location build failures

[Per Bothner]

These patches fix various problems with boostrapping with
--enable-mapped-location.  The orginal patch was posted 02/11.
Mark approved the C++ part, so I've checked that in.
No response on the other parts.

I have added two changes since then:
(1) The c_parser_for_statement is a fix for a local variable that may
be used uninitialized.  I'm guessing the back-end doesn't catch that
when the location_t local is a structure as opposed to an int?
(2) I've ran into a problem dwarf2out.c crashing when passed a not-found
class.  In mainline dwarf2out.c bypasses the class if DECL_IS_BUILTIN -
and a not-found class gets a zero line number.  I think this is a jc1
bug, and I'm looking into it, but for now it's probably easiest to use
the old DECL_IS_BUILTIN definition.

As as inducement:  As soon as I've checked this in, I'll post the
patches for correct column numbers of local variables, for C and C++.
-- 
	--Per Bothner
per@bothner.com   http://per.bothner.com/
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