Add -lit-xml-output-path option to hctbuild.cmd (#5381)

This is used set a path that lit will use to write out an xml file
containing the test results.

I had tried to reuse the existing `-dxc-cmake-extra-args` flag to set
the output, but I ran into a lot of problems getting the arg value to
parse correctly since it has embedded `=` signs that cmd treats as a
command line separator the same as a space (we need to pass a value like
`-DLLVM_LIT_ARGS=--xunit-xml-output=log.xml`). No combination of quotes
and backslashes could get the arg value passed through correctly, so I
opted for the direct route taken in this PR.

utils/hct/hctbuild.cmd

@@ -199,6 +199,11 @@ if "%1"=="-enable-lit" (
   set ENABLE_LIT=ON
   shift /1 & goto :parse_args
 )
+if "%1"=="-lit-xml-output-path" (
+  set "CMAKE_OPTS=%CMAKE_OPTS% -DLLVM_LIT_ARGS=--xunit-xml-output=%~2"
+  shift /1
+  shift /1 & goto :parse_args
+)
 rem Begin SPIRV change
 if "%1"=="-spirv" (
   echo SPIR-V codegen is enabled.