Add a function to MD5 a file's contents.

Add a function to MD5 a file's contents.

In doing so, clean up the MD5 interface a little.  Most
existing users only care about the lower 8 bytes of an MD5,
but for some users that care about the upper and lower,
there wasn't a good interface.  Furthermore, consumers
of the MD5 checksum were required to handle endianness
details on their own, so it seems reasonable to abstract
this into a nicer interface that just gives you the right
value.

Differential Revision: https://reviews.llvm.org/D31105

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298322 91177308-0d34-0410-b5e6-96231b3b80d8