Remove pointless backslash

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@64965 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Headers/tgmath.h

@@ -38,7 +38,7 @@
 // promotion
 
 typedef void _Argument_type_is_not_arithmetic;
-static _Argument_type_is_not_arithmetic __tg_promote(...) \
+static _Argument_type_is_not_arithmetic __tg_promote(...)
   __attribute__((__unavailable__,__overloadable__));
 static double               _TG_ATTRSp __tg_promote(int);
 static double               _TG_ATTRSp __tg_promote(unsigned int);