Remove template arguments from drgn.Type.tag

[osandov]

As noted by 352c31e, drgn.Type.tag includes the template arguments because the DW_AT_name attribute in DWARF includes them. tag should just be what the C standard calls tag and what the C++ standard calls class-head-name: the identifier after the struct, union, or class keyword.

To do this, we need to strip everything starting at the < character when parsing DWARF for a structure, union, or class. But this means that we can't use the string directly in the mmap'd DWARF anymore; we'll have to allocate it and track it to deallocate it (maybe interning it in a set, maybe just keeping them all in a vector). Or, we can edit the mmap, but that might get awkward with the DWARF index.

Then, when formatting a type with template parameters, we need to include those after the tag.