[SYCL][ABI-Break] Hide getPlugin symbol

[AlexeySachkov]

This function is not called anywhere from SYCL headers and therefore there is no reason to have it exported.

It was made exported as part of ESIMD emulator plugin addition (#2963), but that plugin was since
then deprecated and removed (#13295).

This is patch is essentially a by-product of #14145 and it is done to simplify that change, i.e. PI plugins removal should not be ABI-breaking by itself, we just need to cleanup some of our exported symbols.

[aelovikov-intel]

We should drop

llvm/sycl/tools/abi_check.py

Lines 71 to 92 in 7d7ab2f

	# pi.cpp:
	# template <backend BE> const PluginPtr &getPlugin() {
	# static const plugin *Plugin = nullptr;
	# ...
	# }
	# // explicit dllexport instantiations.
	#
	# clang-cl generates exported symbols for the static variables Plugin. These are never referenced
	# in the user's headers so cannot be used outside DLL and not exporting them should not affect any
	# usage scenario.
	#
	# In general, the compiler doesn't know if the definition is in the DLL or in the header and inline
	# dllexport/dllimport functions have to be supported, hence clang-cl's behavior.
	#
	# See also https://devblogs.microsoft.com/oldnewthing/20140109-00/?p=2123.
	ignore_symbols += [
	"?Plugin@?1???$getPlugin@$01@pi@detail@_V1@sycl@@YAAEBVplugin@234@XZ@4PEBV5234@EB",
	"?Plugin@?1???$getPlugin@$00@pi@detail@_V1@sycl@@YAAEBVplugin@234@XZ@4PEBV5234@EB",
	"?Plugin@?1???$getPlugin@$04@pi@detail@_V1@sycl@@YAAEBVplugin@234@XZ@4PEBV5234@EB",
	"?Plugin@?1???$getPlugin@$02@pi@detail@_V1@sycl@@YAAEBVplugin@234@XZ@4PEBV5234@EB",
	"?Plugin@?1???$getPlugin@$05@pi@detail@_V1@sycl@@YAAEBVplugin@234@XZ@4PEBV5234@EB",
	]

as part of this, probably. On the other hand, given the abi breaking window closure we can address it in a separate PR.

[AlexeySachkov]

We should drop

llvm/sycl/tools/abi_check.py

Lines 71 to 92 in 7d7ab2f

	# pi.cpp:
	# template <backend BE> const PluginPtr &getPlugin() {
	# static const plugin *Plugin = nullptr;
	# ...
	# }
	# // explicit dllexport instantiations.
	#
	# clang-cl generates exported symbols for the static variables Plugin. These are never referenced
	# in the user's headers so cannot be used outside DLL and not exporting them should not affect any
	# usage scenario.
	#
	# In general, the compiler doesn't know if the definition is in the DLL or in the header and inline
	# dllexport/dllimport functions have to be supported, hence clang-cl's behavior.
	#
	# See also https://devblogs.microsoft.com/oldnewthing/20140109-00/?p=2123.
	ignore_symbols += [
	"?Plugin@?1???$getPlugin@$01@pi@detail@_V1@sycl@@YAAEBVplugin@234@XZ@4PEBV5234@EB",
	"?Plugin@?1???$getPlugin@$00@pi@detail@_V1@sycl@@YAAEBVplugin@234@XZ@4PEBV5234@EB",
	"?Plugin@?1???$getPlugin@$04@pi@detail@_V1@sycl@@YAAEBVplugin@234@XZ@4PEBV5234@EB",
	"?Plugin@?1???$getPlugin@$02@pi@detail@_V1@sycl@@YAAEBVplugin@234@XZ@4PEBV5234@EB",
	"?Plugin@?1???$getPlugin@$05@pi@detail@_V1@sycl@@YAAEBVplugin@234@XZ@4PEBV5234@EB",
	]

as part of this, probably. On the other hand, given the abi breaking window closure we can address it in a separate PR.

#14637