Modernize the rest of the Find.* API (NFC)

This patch removes the size_t return value and the append parameter
from the remainder of the Find.* functions in LLDB's internal API. As
in the previous patches, this is motivated by the fact that these
parameters aren't really used, and in the case of the append parameter
were frequently implemented incorrectly.

Differential Revision: https://reviews.llvm.org/D69119

llvm-svn: 375160