[flang] fix build on *BSD after 4762c65 (#86204)

The HUGE definition collides with the HUGE macro from math.h. Unlike the
fix in 3149c93 (#84478) (largely reverted in f95710c), add
another #undef HUGE since there is no practical way to make FreeBSD's
headers not define HUGE and still define XSI interfaces such as isascii
or strnlen.

Update comments above `#undef HUGE` instances to reflect the fact that
all major BSD versions (I checked DragonFly, FreeBSD, NetBSD, and
OpenBSD) leak the HUGE macro from math.h to various degrees.

Fixes #86038

flang/include/flang/Evaluate/integer.h

@@ -27,7 +27,7 @@
 #include <string>
 #include <type_traits>
 
-// Some environments, viz. glibc 2.17, allow the macro HUGE
+// Some environments, viz. glibc 2.17 and *BSD, allow the macro HUGE
 // to leak out of <math.h>.
 #undef HUGE
 

flang/include/flang/Evaluate/real.h

@@ -18,7 +18,7 @@
 #include <limits>
 #include <string>
 
-// Some environments, viz. glibc 2.17, allow the macro HUGE
+// Some environments, viz. glibc 2.17 and *BSD, allow the macro HUGE
 // to leak out of <math.h>.
 #undef HUGE
 

flang/lib/Decimal/decimal-to-binary.cpp

@@ -16,6 +16,10 @@
 #include <cstring>
 #include <utility>
 
+// Some environments, viz. glibc 2.17 and *BSD, allow the macro HUGE
+// to leak out of <math.h>.
+#undef HUGE
+
 namespace Fortran::decimal {
 
 template <int PREC, int LOG10RADIX>

flang/lib/Evaluate/fold-implementation.h

@@ -39,7 +39,7 @@
 #include <type_traits>
 #include <variant>
 
-// Some environments, viz. glibc 2.17, allow the macro HUGE
+// Some environments, viz. glibc 2.17 and *BSD, allow the macro HUGE
 // to leak out of <math.h>.
 #undef HUGE