Emit Tag_ABI_FP_16bit_format build attribute.

The __fp16 type is unconditionally exposed. Since -mfp16-format is not yet
supported, there is not a user switch to change this behaviour. This build
attribute should capture the default behaviour of the compiler, which is to
expose the IEEE 754 version of __fp16.

When -mfp16-format is emitted, that will be the way to control the value of
this build attribute.

Change-Id: I8a46641ff0fd2ef8ad0af5f482a6d1af2ac3f6b0

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224115 91177308-0d34-0410-b5e6-96231b3b80d8