[llvm-pdbdump] Support dynamic load address and external symbols.

This patch adds the --load-address command line option to
llvm-pdbdump, which dumps all addresses assuming the module has
loaded at the specified address.

Additionally, this patch adds an option to llvm-pdbdump to support
dumping of public symbols (i.e. symbols with external linkage).

llvm-svn: 236342