optimise readpdb.c filename and dirname handling, following r14158

optimise readpdb.c filename and dirname handling, following r14158

r14158 introduced a dedup pool to store pairs (filename,dirname).
The windows debug info reader (readpdb.c) performance was still to be
improved, as calls to ML_(addFnDn) were done for each line loc to add.

With this patch, the nr of calls to ML_(addFnDn) should be reduced
significantly.

Code has been compiled and regtested on linux, but no windows/wine test
was done.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14183