Breakpad: Refine record classification code

Previously we would classify all STACK records into a single bucket.
This is not really helpful, because there are three distinct types of
records beginning with the token "STACK" (STACK CFI INIT, STACK CFI,
STACK WIN). To be consistent with how we're treating other records, we
should classify these as three different record types.

It also implements the logic to put "STACK CFI INIT" and "STACK CFI"
records into the same "section" of the breakpad file, as they are meant
to be read together (similar to how FUNC and LINE records are treated).

The code which performs actual parsing of these records will come in a
separate patch.

llvm-svn: 357691