[clang] Remove FEM_Indeterminable (#137661)

Remove FEM_Indeterminable as it is unused and cannot be stored safely in
an unsigned bitfield

Author: ojhunt

clang/include/clang/Basic/LangOptions.h

@@ -299,10 +299,7 @@ class LangOptionsBase {
   };
 
   /// Possible float expression evaluation method choices.
-  enum FPEvalMethodKind {
-    /// The evaluation method cannot be determined or is inconsistent for this
-    /// target.
-    FEM_Indeterminable = -1,
+  enum FPEvalMethodKind : unsigned {
     /// Use the declared type for fp arithmetic.
     FEM_Source = 0,
     /// Use the type double for fp arithmetic.