[CodeView Type Merging] Don't keep re-allocating temp serializer.

Previously, every time we wanted to serialize a field list record, we
would create a new copy of FieldListRecordBuilder, which would in turn
create a temporary instance of TypeSerializer, which itself had a
std::vector<> that was about 128K in size. So this 128K allocation was
happening every time. We can re-use the same instance over and over, we
just have to clear its internal hash table and seen records list between
each run. This saves us from the constant re-allocations.

This is worth an ~18.5% speed increase (3.75s -> 3.05s) in my tests.

Differential Revision: https://reviews.llvm.org/D33506

llvm-svn: 303919