[lldb] Fix a 32 bit warning in ScriptedProcessInterface

../llvm-project/lldb/include/lldb/Interpreter/ScriptedProcessInterface.h:61:12:
warning: implicit conversion from 'unsigned long long' to 'size_t' (aka 'unsigned int')
changes value from 18446744073709551615 to 4294967295 [-Wconstant-conversion]
../llvm-project/lldb/source/Plugins/Process/scripted/ScriptedProcess.cpp:275:39:
warning: result of comparison of constant 18446744073709551615 with expression
of type 'size_t' (aka 'unsigned int') is always false [-Wtautological-constant-out-of-range-compare]

This happens because size_t on 32 bit is 32 bit, but LLDB_INVALID_OFFSET is
UINT64_MAX. Return lldb::offset_t instead, which is 64 bit everywhere.

DoWriteMemory still returns size_t but this is because every other
Process derived thing does that. As long as the failure check works I think
it should be fine.

Reviewed By: mib

Differential Revision: https://reviews.llvm.org/D146124