[msan] Better open_memstream support.

Move fflush and fclose interceptors to sanitizer_common.
Use a metadata map to keep information about the external locations
that must be updated when the file is written to.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@208676 91177308-0d34-0410-b5e6-96231b3b80d8