Fix -Wshadow warning in libiberty/cp-demangle.c

Fix -Wshadow warning in libiberty/cp-demangle.c

valgrind contains a copy of the libiberty demangler which gets compiled
with -Wshadow. That shows the following warning:

cp-demangle.c: In function ‘d_substitution’:
cp-demangle.c:3772:35: warning: declaration of ‘c’ shadows a previous local
        struct demangle_component *c;
                                   ^
cp-demangle.c:3708:8: note: shadowed declaration is here
   char c;
        ^

Fix that by renaming the struct demangle_component variable to dc.

libiberty/ChangeLog:

	* cp-demangle.c (d_substitution): Change struct demangle_component
	variable name from c to dc.

From-SVN: r240068