[debuginfo-tests] Fix environment variable used to specify LLDB
Currently, if the user specifies the environment variable 'CLANG', tests
will attempt to use the value as a path to the clang executable.
Previously, lldb could also be specified via the CLANG environment
variable, but this was almost certainly a bug, because that meant both
clang and lldb would have the same path. This patch changes the
environment variable for lldb to 'LLDB'.
Reviewed by: thopre, teemperor
Differential Revision: https://reviews.llvm.org/D101982