[NFC] Fix some warnings and errors under C++20

[Ralender]

Fixed a compile error under C++20.
Fixed multiple warnings and oddities

[elizabethandrews]

The SemaSYCL changes look ok to me, but can you update the PR description with what error/warnings were fixed.

[rbegam]

LGTM. please resolve the conflicts.

[keryell]

Just for some context, this comes from triSYCL/sycl#76 where our own runtime is using C++20.
@Ralender: I have the feeling that you have not addressed all the messages like

llvm/sycl/include/CL/sycl/handler.hpp:572:57: warning: implicit capture of ‘this’ via ‘[=]’ is deprecated in C++20 [-Wdeprecated]
  572 |                                     IsPHSrc, IsPHDst>> ([=]() {

which appear when compiling with g++-9 and g++-10.
And while you are here you can remove the useless () in the lambda.