Clear the LV cache when merging the availability attribute.

The availability implies default visibility, so it can change the computed
visibility.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171840 91177308-0d34-0410-b5e6-96231b3b80d8
docs: "clang tools" are not a different interface.

I spotted someone confused about this, so remove the heading to make it
clear that they really are a part of Tooling.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171841 91177308-0d34-0410-b5e6-96231b3b80d8