Get rid of SanitizerOptions::Disabled global. NFC.

Get rid of SanitizerOptions::Disabled global. NFC.

SanitizerOptions is not even a POD now, so having global variable of
this type, is not nice. Instead, provide a regular constructor and clear()
method, and let each CodeGenFunction has its own copy of SanitizerOptions
it uses.

llvm-svn: 220920