Change the MemoryBuffer::getFile* methods to take just a pointer to the

Change the MemoryBuffer::getFile* methods to take just a pointer to the
start of a filename, not a filename+length.  All clients can produce a
null terminated name, and the system api's require null terminated
strings anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@49041 91177308-0d34-0410-b5e6-96231b3b80d8