[llvm] cmake: Allow CLANG_RESOURCE_DIR to be absolute. (PR #145996)

[via llvm-commits]

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@@ -10,7 +10,7 @@ function(get_clang_resource_dir out_var)
   cmake_parse_arguments(ARG "" "PREFIX;SUBDIR" "" ${ARGN})
 
   if(DEFINED CLANG_RESOURCE_DIR AND NOT CLANG_RESOURCE_DIR STREQUAL "")
-    set(ret_dir bin/${CLANG_RESOURCE_DIR})
+    cmake_path(APPEND bin ${CLANG_RESOURCE_DIR} OUTPUT_VARIABLE ret_dir)
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paperchalice wrote:

`cmake_path(APPEND <path-var> [<input>...] [OUTPUT_VARIABLE <out-var>])`
IIUC here `bin` should be replaced by a variable name:
```suggestion
    set(ret_dir bin)
    cmake_path(APPEND ret_dir ${CLANG_RESOURCE_DIR})
```

https://github.com/llvm/llvm-project/pull/145996