[SYCL][Driver] Emit an error when PCH is triggered in SYCL mode.

[srividya-sundaram]

Emit an error if PCH(Pre-Compiled Header) file generation is forced in -fsycl mode.

[mdtoguchi]

As I think about this a little more, the use model for creating a PCH file is more implied and may catch folks off guard. Let's say a person creates a PCH file with a non-SYCL compilation with clang++ my_header.h. Later on, they want to pull in that header with an -fsycl compile like clang++ -fsycl -include my_header.h file.cpp. I don't see this as being invalid. So when we go into consumption of PCH files with -fsycl we should think about this case.

[srividya-sundaram]

@intel/llvm-gatekeepers Hi, this is ready to be merged.

[srividya-sundaram]

@intel/llvm-gatekeepers ping.
The failures in SYCL Pre Commit / Linux / [AWS] CUDA LLVM Test Suite (pull_request_target) is unrelated to the changes in this PR.