Make `this` captures explicit for C++20; NFC (#3763)
Although LLVM has no immediate plans to switch to C++20, prepare for the
deprecation of implicit capture of `this` through `[=]` in lambdas.
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/a81d73ed19f9a9d
