[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

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224736 91177308-0d34-0410-b5e6-96231b3b80d8