Preserve LD_LIBRARY_PATH when using the 'env' command

In many Linux environments (and similar), just-built applications won't run
correctly without making use of the current LD_LIBRARY_PATH environmental
variable in order to find dynamic libraries. Propagate it through the 'env'
command (hopefully this works on all platforms).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223219 91177308-0d34-0410-b5e6-96231b3b80d8