[libunwind] [runtimes] Fix parsing of LIB{CXX,CXXABI,UNWIND}_TEST_PARAMS (PR #67691)

[Louis Dionne via cfe-commits]

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@@ -0,0 +1,20 @@
+
+macro(serialize_lit_param param value)
+  string(APPEND SERIALIZED_LIT_PARAMS "config.${param} = ${value}\n")
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ldionne wrote:

This looks reasonable, but could we avoid using the global variable `SERIALIZED_LIT_PARAMS` here? We should thread it through the functions instead, otherwise these functions all "return their result" by modifying the global variable, which is yucky.

https://github.com/llvm/llvm-project/pull/67691