[Sanitizer] Refactor CommonFlags interface. NFC.

Add CommonFlags::SetDefaults() and CommonFlags::ParseFromString(),
so that this object can be easily tested. Enforce
that ParseCommonFlagsFromString() and SetCommonFlagsDefaults()
work only with singleton CommonFlags, shared across all sanitizer
runtimes.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224617 91177308-0d34-0410-b5e6-96231b3b80d8