When we're deserializing declarations lexically stored in a RecordDecl

after having already deserialized the fields, clear out the fields
first. This makes sure that we keep all of the declarations in the
lexical context (including those implicitly added by later
type-checking) within the same list. A test case for this behavior is
coming as part of another commit; testing for this problem in
isolation is a nightmare.

llvm-svn: 138661