typeinfo: provide a partial implementation for Win32

The RTTI structure is different on Windows when building under MS ABI.
Update the definition to reflect this. The structure itself contains an
area for caching the undecorated name (which is 0-initialized). The
decorated name has a bitfield followed by the linkage name. When
std::type_info::name is invoked for the first time, the runtime should
undecorate the name, cache it, and return the undecorated name. This
requires access to an implementation of __unDName. For now, return
the raw name.

This uses the fnv-1a hash to hash the name of the RTTI. We could use an
alternate hash (murmur? city?), but, this was the quickest to throw
together.

llvm-svn: 313344