[libclc] [libclc] Fix building top-level 'libclc' target (PR #150972)

[Wenju He via cfe-commits]

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@@ -5,6 +5,9 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
 endif()
 set(LLVM_SUBPROJECT_TITLE "libclc")
 
+# Top level target used to build all Libclc libraries.
+add_custom_target( libclc ALL )
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wenju-he wrote:

can we put this line near line 48~49, or after line 11? Setting CMAKE_CXX_STANDARD is a setup of the project and is likely better placed before defining a target.

https://github.com/llvm/llvm-project/pull/150972