Fix the predefined exponent limit macros for the 16-bit IEEE format.

The magnitude range of normalized _Float16 is 2^-14 (~6e-5) to
(2-2^-10)*2^15 (65504).  You might think, then, that the code is
correct to defne FLT16_MIN_EXP and FLT16_MAX_EXP to be -14 and 15
respectively.  However, for some reason the C specification actually
specifies a bias for these macros:

C11 5.2.4.2.2:

  - minimum negative integer such that FLT_RADIX raised to one less than
    that power is a normalized floating-point number, e_min:
      FLT_MIN_EXP
      DBL_MIN_EXP
      LDBL_MIN_EXP

  - maximum integer such that FLT_RADIX raised to one less than that
    power is a representable finite floating-point number, e_max:
      FLT_MAX_EXP
      DBL_MAX_EXP
      LDBL_MAX_EXP

FLT16_MIN_EXP and FLT16_MAX_EXP should clearly be biased the same way,
and other compilers do in fact do so, as do our OpenCL headers for `half`.

Additionally, FLT16_MIN_10_EXP is just wrong.

llvm-svn: 362183