Add concrete type overloads to PDBSymbol::findChildren().

Frequently you only want to iterate over children of a specific
type (e.g. functions).  Previously you would get back a generic
interface that allowed iteration over the base symbol type,
which you would have to dyn_cast<> each one of.  With this patch,
we allow the user to specify the concrete type as a template
parameter, and it will return an iterator which returns instances
of the concrete type directly.

llvm-svn: 228960