ExternalData: Preserve escaped semicolons during argument expansion

The CMake language implicitly flattens lists so a ";" in a list element
must be escaped with a backslash.  List expansion removes backslashes
escaping semicolons to leave raw semicolons in the values.  Teach
ExternalData_Add_Test and ExternalData_Expand_Arguments to re-escape
semicolons found in list elements so the resulting argument lists work
as if constructed directly by the set() command.

For example:

  ExternalData_Add_Test(Data NAME test1 COMMAND ... "a\\;b")
  ExternalData_Expand_Arguments(Data args2 "c\\;d")
  add_test(NAME test2 COMMAND ... ${args2})

should be equivalent to

  set(args1 "a\\;b")
  add_test(NAME test1 COMMAND ... ${args1})
  set(args2 "c\\;d")
  add_test(NAME test2 COMMAND ... ${args2})

which is equivalent to

  add_test(NAME test1 COMMAND ... "a;b")
  add_test(NAME test2 COMMAND ... "c;d")

Note that it is not possible to make ExternalData_Add_Test act exactly
like add_test when quoted arguments contain semicolons because the CMake
language flattens lists when constructing function ARGN values.  This
re-escape approach at least allows test arguments to have semicolons.

While at it, teach ExternalData APIs to not transform "DATA{...;...}"
arguments because the contained semicolons are non-sensical.

Suggested-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

Tests/Module/ExternalData/CMakeLists.txt

@@ -35,6 +35,7 @@ ExternalData_Add_Test(Data1
     -D Paired=DATA{PairedA.dat,PairedB.dat}
     -D Meta=DATA{MetaTop.dat,REGEX:Meta[ABC].dat}
     -D Directory=DATA{Directory/,A.dat,REGEX:[BC].dat}
+    -D "Semicolons=DATA{Data.dat}\\;DATA{Data.dat}"
     -P ${CMAKE_CURRENT_SOURCE_DIR}/Data1Check.cmake
   )
 ExternalData_Add_Target(Data1)

Tests/Module/ExternalData/Data1Check.cmake

@@ -56,3 +56,13 @@ foreach(n A B C)
     message(SEND_ERROR "Input file:\n  ${file}\ndoes not exist!")
   endif()
 endforeach()
+list(LENGTH Semicolons len)
+if("${len}" EQUAL 2)
+  foreach(file ${Semicolons})
+    if(NOT EXISTS "${file}")
+      message(SEND_ERROR "Input file:\n  ${file}\ndoes not exist!")
+    endif()
+  endforeach()
+else()
+  message(SEND_ERROR "Semicolons value:\n  ${Semicolons}\nis not a list of length 2.")
+endif()