[msan] Transform memcpy and memset to library calls.
[msan] Transform memcpy and memset to library calls.

This was already done for memmove, where it is required for correctness.
This change improves performance by avoiding copyingthe same memory twice.
Also, the library functions are given __msan_ prefix to prevent instcombine
pass from converting them back to intrinsics.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@168876 91177308-0d34-0410-b5e6-96231b3b80d8

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