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ASan allocates a global data initialization array at the tail end of each

Description

ASan allocates a global data initialization array at the tail end of each
compunit's .data section. This vector is not poisoned. Because of this the
first symbol of the following section has no left red zone. As a result, ASan
cannot detect underflow for such symbols.

Poison ASan allocated metadata, it should not be accessible to user code.

This fix does not eliminate the problem with missing left red zones but it
reduces the set of vulnerable symbols from first symbols in each input data
section to first symbols in the output section of the binary.

Differential Revision: https://reviews.llvm.org/D38056