[CodeView] Truncate display names

Fundamentally, the length of a variable or function name is bound by the
maximum size of a record: 0xffff.  However, the name doesn't live in a
vacuum; other data is associated with the name, lowering the bound
further.

We would naively attempt to emit the name, causing us to assert because
the record would no-longer fit in 16-bits.  Instead, truncate the name
but preserve as much as we can.

While I have tested this locally, I've decided to not commit it due to
the test's size.

N.B.  While this behavior is undesirable, it is better than MSVC's
behavior.  They seem to truncate to ~4000 characters.

llvm-svn: 263378