[CMake] Add an option to disable source generation (microsoft#6730)

This change adds a new CMake configuration option
`HLSL_DISABLE_SOURCE_GENERATION` which allows a user to disable
generating the in-tree sources which contributte to DXC's source
releases. This option should only be used by users building DXC and not
modifying it.

Resolves microsoft#6728

Author: llvm-beanz

cmake/modules/HCT.cmake

@@ -1,4 +1,6 @@
 option(HLSL_COPY_GENERATED_SOURCES "Copy generated sources if different" Off)
+option(HLSL_DISABLE_SOURCE_GENERATION "Disable generation of in-tree sources" Off)
+mark_as_advanced(HLSL_DISABLE_SOURCE_GENERATION)
 
 add_custom_target(HCTGen)
 
@@ -43,6 +45,10 @@ function(add_hlsl_hctgen mode)
   if (NOT ARG_OUTPUT)
     message(FATAL_ERROR "add_hlsl_hctgen requires OUTPUT argument")
   endif()
+
+  if (HLSL_DISABLE_SOURCE_GENERATION AND NOT ARG_BUILD_DIR)
+    return()
+  endif()
 
   set(temp_output ${CMAKE_CURRENT_BINARY_DIR}/tmp/${ARG_OUTPUT})
   set(full_output ${CMAKE_CURRENT_SOURCE_DIR}/${ARG_OUTPUT})