[clang] [analyzer] Compute length of string literal initializers (#66990) (PR #68368)

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luamfb wrote:

I've added that test as well. In this case too I've assumed the correct behavior is for the comparison's result to be unknown, because the length of the string can be changed later by assigning a null character somewhere in the middle of the string, say, `char a[] = "abcd"; a[1] = '\0';`

https://github.com/llvm/llvm-project/pull/68368