Make predefined FLT16 macros conditional on support for the type

We unconditionally predefine these macros. However, they may be used to
determine if the type is supported. In that case, there are unnecessary
failures to compile the code.

This is the proposed fix for https://bugs.llvm.org/show_bug.cgi?id=40559

Differential revision: https://reviews.llvm.org/D57577

llvm-svn: 354512