Fix the value of Float80.pi (#12881)

Currently this is implemented as Float80(literal), which gets interpreted as Float80(Double(literal)), unfortunately. Switch to the correct spelling: literal as Float80.

Resolves rdar://problem/35459284

Author: stephentyrone

stdlib/public/core/FloatingPointTypes.swift.gyb

@@ -559,11 +559,11 @@ extension ${Self}: BinaryFloatingPoint {
     // in angles in the wrong quadrant if users aren't careful.  This is
     // not a problem for Double or Float80, as pi rounds down in both of
     // those formats.
-    return Float(0x1.921fb4p1)
+    return 0x1.921fb4p1
 %elif bits == 64:
-    return Double(0x1.921fb54442d18p1)
+    return 0x1.921fb54442d18p1
 %elif bits == 80:
-    return Float80(0x1.921fb54442d1846ap1)
+    return 0x1.921fb54442d1846ap1
 %end
   }