[Sanitizer] Make CommonFlags immutable after initialization.
Summary:
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.
Test Plan: regression test suite
Reviewers: kcc, eugenis, glider
Subscribers: llvm-commits
Differential Revision: http://reviews.llvm.org/D6741
llvm-svn: 224736