Allow to disable all sanitizers with "-fno-sanitize=all" option.

Summary:
This patch adds "all" sanitizer group. A shortcut "-fno-sanitize=all"
can be used to disable all sanitizers for a given source file.

"-fsanitize=all" option makes no sense, and will produce an error.

This group can also be useful when we add "-fsanitize-recover=<list>"
options (patch in http://reviews.llvm.org/D6302), as it would allow
to conveniently enable/disable recovery for all specified sanitizers.

Test Plan: regression test suite

Reviewers: kcc, rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6733