[llvm] [APFloat] Add `APFloat::getConstant` for C++20 <numbers> constants (PR #227916)

[Matt Arsenault via llvm-commits]

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@@ -6061,6 +6061,53 @@ APFloat APFloat::getAllOnesValue(const fltSemantics &Semantics) {
   return APFloat(Semantics, APInt::getAllOnes(Semantics.sizeInBits));
 }
 
+// The same literals as <numbers>, which are of sufficient precision for f128
+// and ppcf128. These literals will need to be updated if we add support for
+// f256 in the future.
+static constexpr StringLiteral MathConstantStrings[] = {
+    "-2.718281828459045235360287471352662498", // e
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arsenm wrote:

I would hope to store these as some constant the APFloat can directly initialize to without parsing 

https://github.com/llvm/llvm-project/pull/227916