Use memmove in PODArray::insert to handle memory overlapping.

[fuzhe1989]

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PODArray::insert will move a bunch of data forward to leave space for new items. The new memory range can be overlapping with the old memory range when bytes_to_move is larger than bytes_to_copy and use of memcpy here has undefined behavior.

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Use memmove in PODArray::insert to handle memory overlapping.

[alexey-milovidov]

We need to cover this case in unit test (see gtest_pod_array.cpp)

[fuzhe1989]

Hi alexey, from spec memcpy's behavior is undefined if the src and dst range are overlapping. However from here:

on Linux, up until fairly recently, the implementation of memcpy() worked in a way that was similar enough to memmove() that overlapping memory wasn't an issue.

And I tested locally they have the same behavior (gcc-9, Ubuntu-12.04). I wonder how to cover this case while avoiding intrusive code change (e.g. do some tricks under NDEBUG), do you have any idea?

[kitaisreal]

@fuzhe1989 actually asan can check memcpy overlap regions, will update gtest_pod_array to check that case.