CodeGen: update comment about RTTI field

The MS ABI RTTI has a reserved field which is used as a cache for the
demangled name.  It must be zero-initialized, which is used as a hint by
the runtime to say that the cache has not been populated.  Since this
field is populated at runtime, the RTTI structures must be placed in the
.data section rather than .rdata.  NFC

llvm-svn: 290799