[Sanitizer] Turn SuppressionContext::Init() into InitIfNecessary().

Suppression context might be used in multiple sanitizers working
simultaneously (e.g. LSan and UBSan) and not knowing about each other.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@214831 91177308-0d34-0410-b5e6-96231b3b80d8