typeinfo: style adjustments for adding MS ABI RTTI

This is motivated by adding a third RTTI scheme to libc++.  Split out
the two forms of the itanium RTTI representation.  This is based on
suggestions from Eric Fiselier.  NFC

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291174 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed