[CMake] Remove set(CACHE) line for SWIFT_MODULE_ABI_NAME_PREFIX

Due to https://cmake.org/cmake/help/latest/policy/CMP0126.html
set(CACHE) overwrites the existing value if the cache has not been set
to any value. Since we don't need any default value, just remove it.

(cherry picked from commit d8c8695)

Author: rintaro

CMakeLists.txt

@@ -15,8 +15,6 @@ project(SwiftSyntax LANGUAGES C Swift)
 set(SWIFT_VERSION 5)
 set(CMAKE_Swift_LANGUAGE_VERSION ${SWIFT_VERSION})
 
-set(SWIFT_MODULE_ABI_NAME_PREFIX CACHE STRING "ABI name prefix to avoid name conflicts")
-
 # The subdirectory into which host libraries will be installed.
 set(SWIFT_HOST_LIBRARIES_SUBDIRECTORY "swift/host")
 

cmake/modules/AddSwiftHostLibrary.cmake

@@ -56,6 +56,7 @@ function(add_swift_host_library name)
       -emit-module-interface-path;${module_interface_file}
     >)
   if(SWIFT_MODULE_ABI_NAME_PREFIX)
+    # ABI name prefix. this can be used to avoid name conflicts.
     target_compile_options("${name}" PRIVATE
       $<$<COMPILE_LANGUAGE:Swift>:
         "SHELL:-Xfrontend -module-abi-name"