Introduce a new libclang API to determine the parent context of a code

Introduce a new libclang API to determine the parent context of a code
completion item. For example, if the code completion itself represents
a declaration in a namespace (say, std::vector), then this API
retrieves the cursor kind and name of the namespace (std). Implements
<rdar://problem/11121951>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@153545 91177308-0d34-0410-b5e6-96231b3b80d8