[SYCL] Add temporary flag for PI link

The previous commit adds new logic to the Level Zero implementation of
`piProgramLink`, but this logic is only used currently if the SYCL
application explicitly calls `sycl::link`.  When an application merely
executes a kernel, the SYCL runtime generally calls `piProgramBuild`
instead of `piProgramCompile` / `piProgramLink`.

In order to enable wider testing of the new `piProgramLink` logic, this
commit adds a temporary environment variable which forces the SYCL
runtime to build all programs by calling `piProgramCompile` /
`piProgramLink`.  This is usually safe, though there are a few cases
where it is unsafe and results in false errors.  Therefore, use this
environment variable with caution.

Usage is like:

```
SYCL_FORCE_LINK=[0|1]
```

When running tests with `llvm-lit`, you can set this environment
variable by passing `--param extra_environment=SYCL_FORCE_LINK=1`:

```
$ llvm-lit --param extra_environment=SYCL_FORCE_LINK=1  \
  --param sycl_be=level_zero --param target_devices=gpu \
  -j12 -v -s <test>
```

Here are the known cases when specifying `SYCL_FORCE_LINK=1` results in
a false error:

* Tests using AOT (ahead-of-time compilation) generally fail because
  a `pi_program` that is created from native code cannot be compiled
  with `piProgramCompile`.

* Some tests enable tracing with `SYCL_PI_TRACE` and explicitly check
  for a call to `piProgramBuild`.  These tests fail because the runtime
  calls `piProgramCompile` / `piProgramLink` instead of calling
  `piProgramBuild`.

Author: gmlueck

sycl/source/detail/program_manager/program_manager.cpp

@@ -963,8 +963,11 @@ ProgramManager::ProgramPtr ProgramManager::build(
                                         DeviceLibReqMask);
   }
 
+  static const char *ForceLinkEnv = std::getenv("SYCL_FORCE_LINK");
+  static bool ForceLink = ForceLinkEnv && (*ForceLinkEnv == '1');
+
   const detail::plugin &Plugin = Context->getPlugin();
-  if (LinkPrograms.empty()) {
+  if (LinkPrograms.empty() && !ForceLink) {
     RT::PiResult Error = Plugin.call_nocheck<PiApiKind::piProgramBuild>(
         Program.get(), /*num devices =*/1, &Device, CompileOptions.c_str(),
         nullptr, nullptr);