VS: Fix using PCH from source with COMPILE_OPTIONS

If a source file gets per-source flags from both PCH and custom
`COMPILE_OPTIONS`, combine them correctly.

Fixes: #20694, #20456

Source/cmVisualStudio10TargetGenerator.cxx

@@ -2345,7 +2345,7 @@ void cmVisualStudio10TargetGenerator::OutputSourceSpecificFlags(
         pchOptions =
           this->GeneratorTarget->GetPchUseCompileOptions(config, lang);
       }
-      customAndPchOptions += pchOptions;
+      customAndPchOptions = cmStrCat(customAndPchOptions, ';', pchOptions);
     }
 
     // if we have flags or defines for this config then

Tests/RunCMake/PrecompileHeaders/PchInterface.cmake

@@ -9,6 +9,9 @@ target_precompile_headers(foo PUBLIC
   <stdio.h>
   \"string.h\"
 )
+if(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
+  set_property(SOURCE foo.c APPEND PROPERTY COMPILE_OPTIONS "-WX-")
+endif()
 
 add_library(bar INTERFACE)
 target_include_directories(bar INTERFACE include)