[unittests] Preserve LD_LIBRARY_PATH in crash recovery test

We need to preserve the LD_LIBRARY_PATH environment variable when
spawning a child process (certain setups rely on non-standard paths
for e.g. libstdc++). In order to achieve this, set
LLVM_CRC_UNIXCRCRETURNCODE in the parent process instead of creating
the child's environment from scratch.

Reviewed By: aganea

Differential Revision: https://reviews.llvm.org/D88308