[libc][NFC] Add cmake facility to unify the way we parse arguments.

libc/cmake/modules/LibcParseArguments.cmake

@@ -0,0 +1,87 @@
+set(LLVM_LIBC_OPTIONAL_ARGS
+  ALIAS
+  PUBLIC
+  NO_GPU_BUNDLE
+  NO_RUN_POSTBUILD
+  C_TEST
+  NEED_MPFR
+  NEED_MPC
+  IS_GPU_BENCHMARK
+)
+
+set(LLVM_LIBC_SINGLE_VALUE_ARGS
+  CXX_STANDARD
+  SUITE
+  CREATE_TARGET_FUNCTION
+  HDR
+  DEST_HDR
+  YAML_FILE
+  GEN_HDR
+  NAME
+)
+
+set(LLVM_LIBC_MULTI_VALUE_ARGS
+  HDRS
+  SRCS
+  COMPILE_OPTIONS
+  LINK_OPTIONS
+  LINK_LIBRARIES
+  ENV
+  DEPENDS
+  FLAGS
+  ARGS
+  LOADER_ARGS
+)
+
+foreach(arg_list LLVM_LIBC_OPTIONAL_ARGS LLVM_LIBC_SINGLE_VALUE_ARGS LLVM_LIBC_MULTI_VALUE_ARGS)
+  list(TRANSFORM ${arg_list}
+    PREPEND "OVERLAY_"
+    OUTPUT_VARIABLE ${arg_list}_OVERLAY
+  )
+  list(TRANSFORM ${arg_list}
+    PREPEND "FULL_BUILD_"
+    OUTPUT_VARIABLE ${arg_list}_FULL_BUILD
+  )
+  set(${arg_list}_COMPLETE ${${arg_list}} ${${arg_list}_OVERLAY} ${${arg_list}_FULL_BUILD})
+endforeach()
+
+macro(llvm_libc_parse_arguments name_prefix)
+  cmake_parse_arguments(
+    ${name_prefix}
+    "${LLVM_LIBC_OPTIONAL_ARGS_COMPLETE}"
+    "${LLVM_LIBC_SINGLE_VALUE_ARGS_COMPLETE}"
+    "${LLVM_LIBC_MULTI_VALUE_ARGS_COMPLETE}"
+    ${ARGN}
+  )
+
+  # Collect overlay and full build args
+  foreach(argument IN LISTS LLVM_LIBC_OPTIONAL_ARGS LLVM_LIBC_SINGLE_VALUE_ARGS LLVM_LIBC_MULTI_VALUE_ARGS)
+    if(LLVM_LIBC_FULL_BUILD)
+      if(${name_prefix}_${argument}_FULL_BUILD)
+        list(APPEND ${name_prefix}_${argument} ${${name_prefix}_${argument}_FULL_BUILD})
+      endif()
+    else()
+      if(${name_prefix}_${argument}_OVERLAY)
+        list(APPEND ${name_prefix}_${argument} ${${name_prefix}_${argument}_OVERLAY})
+      endif()
+    endif()
+  endforeach()
+endmacro()
+
+# Forward all arguments that can be used for llvm_libc_parse_arguments again.
+# Assume that *_OVERLAY and *_FULL_BUILD args have been merged properly.
+macro(forward_arguments name_prefix output)
+  set(${output} "")
+
+  foreach(argument ${LLVM_LIBC_OPTIONAL_ARGS})
+    if(${name_prefix}_${argument})
+      list(APPEND output ${argument})
+    endif()
+  endforeach()
+
+  foreach(argument ${LLVM_LIBC_SINGLE_VALUE_ARGS} ${LLVM_LIBC_MULTI_VALUE_ARGS})
+    if(${name_prefix}_${argument})
+      list(APPEND output ${argument} "${${name_prefix}_${argument}}")
+    endif()
+  endforeach()
+endmacro()