[libclang] Use lambdas instead of explicit structs when storing arguments.

[libclang] Use lambdas instead of explicit structs when storing arguments.

This boilerplate code was necessary to move arguments between threads in
C++98, lambdas make this much easier. No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@243227 91177308-0d34-0410-b5e6-96231b3b80d8