[Feature request] Implementation of `textDocument/completion`

[gabrielmfern]

Currently, only the VS Code extension for OpenCL has the implementation for this capability and it would be nice to have it here as well.

[Galarius]

Implementing this capability effectively necessitates constructing an Abstract Syntax Tree (AST) for robust code representation and analysis. Given its existing use in Clang for OpenCL code parsing, libclang emerges as an ideal candidate for this task. However, two primary concerns pose potential challenges:

1. The results produced by utilizing clang_parseTranslationUnit and clang_codeCompleteAt from libclang are not equivalent to those generated by clang -cc1 -code-completion-at file.cl:line:column file.cl for OpenCL.

2. The introduction of libclang results in a substantial dependency. Unfortunately, there are no existing Conan 2.0-compatible recipes (for package management), which adds to the complexity.