Summary:
This is unfortunate, but would let us land http://reviews.llvm.org/D10467,
that makes ToolChains responsible for computing the set of sanitizers
they support.

Unfortunately, Darwin ToolChains doesn't know about actual OS they
target until ToolChain::TranslateArgs() is called. In particular, it
means we won't be able to construct SanitizerArgs for these ToolChains
before that.

This change removes SanitizerArgs::needsLTO() method, so that now
ToolChain::IsUsingLTO(), which is called very early, doesn't need
SanitizerArgs to implement this method.

Docs and test cases are updated accordingly. See
https://llvm.org/bugs/show_bug.cgi?id=23539, which describes why we
start all these.

Test Plan: regression test suite

Reviewers: pcc

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D10560

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240170 91177308-0d34-0410-b5e6-96231b3b80d8