[libc] Clear all exceptions before setting in fesetexceptflag.

[sivachandra]

Summary

Previously, exceptions from the flag were being added. This patch
changes it such that only the exceptions in the flag will be set.

libc/src/fenv/fesetexceptflag.cpp

 
 LLVM_LIBC_FUNCTION(int, fesetexceptflag,
                    (const fexcept_t *flagp, int excepts)) {
   // Since the return type of fetestexcept is int, we ensure that fexcept_t
   // can fit in int type.
   static_assert(sizeof(int) >= sizeof(fexcept_t),
                 "fexcept_t value cannot fit in an int value.");
   int excepts_to_set = static_cast<const int>(*flagp) & excepts;
+  fputil::clearExcept(FE_ALL_EXCEPT);
   return fputil::setExcept(excepts_to_set);
 }
 
 } // namespace __llvm_libc

libc/test/src/fenv/exception_flags_test.cpp

for (int e : excepts) {
     ASSERT_EQ(__llvm_libc::fputil::testExcept(FE_ALL_EXCEPT) & e, 0);
 
     ASSERT_EQ(__llvm_libc::fesetexceptflag(&eflags, FE_ALL_EXCEPT), 0);
     ASSERT_NE(__llvm_libc::fputil::testExcept(FE_ALL_EXCEPT) & e, 0);
 
     // Cleanup
     __llvm_libc::fputil::clearExcept(e);
   }
+
+  // Next, we will raise one exception and save the flags.
+  __llvm_libc::fputil::raiseExcept(FE_INVALID);
+  fexcept_t eflags;
+  __llvm_libc::fegetexceptflag(&eflags, FE_ALL_EXCEPT);
+  // Clear all exceptions and raise two other exceptions.
+  __llvm_libc::fputil::clearExcept(FE_ALL_EXCEPT);
+  __llvm_libc::fputil::raiseExcept(FE_OVERFLOW | FE_INEXACT);
+  // When we set the flags and test, we should only see FE_INVALID.
+  __llvm_libc::fesetexceptflag(&eflags, FE_ALL_EXCEPT);
+  EXPECT_EQ(__llvm_libc::fputil::testExcept(FE_ALL_EXCEPT), FE_INVALID);
 }