[llvm-symbolizer] Support reading options from environment

llvm-symbolizer is used by sanitizers to symbolize errors discovered by
sanitizer, but there's no way to pass options to llvm-symbolizer since
the tool is invoked directly by the sanitizer runtime. Therefore, we
don't have a way to pass options needed to find debug symbols such as
-dsym-hint or -debug-file-directory. This change enables reading options
from the LLVM_SYMBOLIZER_OPTS in addition to command line which can be
used to pass those additional options to llvm-symbolizer invocations
made by sanitizer runtime.

Differential Revision: https://reviews.llvm.org/D71668