Add @_semantics(constant_evaluable) to FixedWidthInteger.bitwidth

Needed for OSLogOptimization to look through the call to this function in
cases it isn't inlined.

rdar://148256435

Author: meg-gupta

stdlib/public/core/Integers.swift

@@ -2158,6 +2158,7 @@ where Magnitude: FixedWidthInteger & UnsignedInteger,
 
 extension FixedWidthInteger {
   @inlinable
+  @_semantics("constant_evaluable")
   public var bitWidth: Int { return Self.bitWidth }
 
   @inlinable