libclang: pass return code out argument by reference

r212427 formalized the message-passing pattern by making these argument
structures const. This commit changes output arguments to get passed by
reference so we can eliminate mutable fields.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212497 91177308-0d34-0410-b5e6-96231b3b80d8