Fix support of openmp for visual studio with clang toolset.

Author: Jarod42

modules/vstudio/tests/vc2019/test_compile_settings.lua

@@ -79,6 +79,20 @@
 		]]
 	end
 
+	function suite.openmpOnWithClang()
+		toolset "clang"
+		openmp "On"
+		prepare()
+		test.capture [[
+<ClCompile>
+	<PrecompiledHeader>NotUsing</PrecompiledHeader>
+	<WarningLevel>Level3</WarningLevel>
+	<Optimization>Disabled</Optimization>
+	<OpenMPSupport>true</OpenMPSupport>
+	<AdditionalOptions>/openmp %(AdditionalOptions)</AdditionalOptions>
+		]]
+	end
+
 --
 -- Check ClCompile for ScanForModuleDependencies
 --

modules/vstudio/vs2010_vcxproj.lua

@@ -1645,6 +1645,10 @@
 		elseif _ACTION >= "vs2019" and cfg.toolset and cfg.toolset == "clang" then
 			local value = iif(cfg.unsignedchar, "On", "Off")
 			table.insert(opts, p.tools.msc.shared.unsignedchar[value])
+			-- <OpenMPSupport>true</OpenMPSupport> is unfortunately ignored with clang toolset
+			if cfg.openmp == "On" then
+				table.insert(opts, 1, '/openmp')
+			end
 		end
 
 		if #opts > 0 then