[libc] [llvm] [libc][math] Implement nan(f|l) functions (PR #76690)

[via llvm-commits]

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@@ -1055,6 +1056,27 @@ hexadecimal_string_to_float(const char *__restrict src,
   return output;
 }
 
+LIBC_INLINE uint64_t
+nan_mantissa_from_ncharseq(const cpp::string_view ncharseq) {
+  uint64_t nan_mantissa = 0;
+
+  if (ncharseq.data() != nullptr && isdigit(*ncharseq.data())) {
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michaelrj-google wrote:

nit: `*ncharseq.data()` should be `ncharseq[0]`

https://github.com/llvm/llvm-project/pull/76690