IRSL: Update redist directory for VS 2019 update 1

VS 2019 Update 1 will fix its redist directories to be named `VC142`
instead of `VC141`.  It will also use cl `19.21` instead of `19.20`
so we can use that to distinguish the versions.

Fixes: #19131

Modules/InstallRequiredSystemLibraries.cmake

@@ -213,10 +213,13 @@ if(MSVC)
   elseif(MSVC_VERSION_VERSION GREATER_EQUAL 143)
     message(WARNING "MSVC toolset v${MSVC_VERSION_VERSION} not yet supported.")
   elseif(MSVC_TOOLSET_VERSION EQUAL 142)
-    # FIXME: VS 2019 RC 4 uses VC141 but an update will fix it to be VC142.
-    set(MSVC_REDIST_NAME VC141)
+    set(MSVC_REDIST_NAME VC142)
     set(_MSVC_DLL_VERSION 140)
     set(_MSVC_IDE_VERSION 16)
+    if(MSVC_VERSION EQUAL 1920)
+      # VS2019 named this differently prior to update 1.
+      set(MSVC_REDIST_NAME VC141)
+    endif()
   elseif(MSVC_TOOLSET_VERSION EQUAL 141)
     set(MSVC_REDIST_NAME VC141)
     set(_MSVC_DLL_VERSION 140)