[Sanitizer] Simplify Symbolizer creation interface.

Get rid of Symbolizer::Init(path_to_external) in favor of
thread-safe Symbolizer::GetOrInit(), and use the latter version
everywhere. Implicitly depend on the value of external_symbolizer_path
runtime flag instead of passing it around manually.

No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@214005 91177308-0d34-0410-b5e6-96231b3b80d8