Revert "Revert r224736: "[Sanitizer] Make CommonFlags immutable after initialization.""

Fix test failures by introducing CommonFlags::CopyFrom() to make sure
compiler doesn't insert memcpy() calls into runtime code.

Original commit message:
Protect CommonFlags singleton by adding const qualifier to
common_flags() accessor. The only ways to modify the flags are
SetCommonFlagsDefaults(), ParseCommonFlagsFromString() and
OverrideCommonFlags() functions, which are only supposed to be
called during initialization.

llvm-svn: 225088