Add _Float128 as alias to __float128 to enable compilations on Fedora27/glibc2-26

Fedora27 is using a new version of glibc that refers to the _Float128 type. This
patch adds that name as an alias to float128. I also added some predefined macro
values for the digits, mantissa, epilon, etc (FloatMacros). For the test case, I
copied an existing float128 test. This functionality needs work long term, but 
it should be sufficient to tread water for a while. At Intel we have test
servers running our LLVM compiler with various open source workloads, the server
has been upgraded to Fedora27 so many workloads are failing due to _Float128.

Patch-By: mibintc
Differential Revision: https://reviews.llvm.org/D40673

llvm-svn: 319703