Add llvm-pdbutil tool_patterns in lit.cfg

This means that 'llvm-pdbutil' in test commands will resolve
to the absolute path to the tool, in line with what happens
already for other tools.

This works either way because the bin directory is also
prepended to the PATH.  I'm not sure why both methods are
used.

Differential Revision: https://reviews.llvm.org/D34128

llvm-svn: 305297