Add llvm-pdbdump to the tool substitutions list in lit. NFC.

Add llvm-pdbdump to the tool substitutions list in lit.  NFC.

This adds llvm-pdbdump to the list of tools which get printed with
the full path in verbose mode.  This makes it easier to take the
whole run line from verbose output and run it again without prepending
with the builds bin directory.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268250 91177308-0d34-0410-b5e6-96231b3b80d8