Reimplement MemoryBuffer::getFile with three enhancements:

1) stop using MappedFile.
2) if profitable use the sys::path::MapInFilePages api to
   read the file.
3) otherwise fallback to read.

When sys::path::MapInFilePages is implemented, this provides
several benefits:

#1: this avoids fragmenting memory for small files.
#2: this avoids extraneous stat calls when the file size is known.
#3: this only keeps the file descriptor open while reading the 
    file, not for the duration of the lifetime of the memory 
    buffer.  This fixes a serious clang FD 'leak' problem.

I believe that this will work on a win32 machine, but I don't have
one to test on.  I'd appreciate it if someone could check.

llvm-svn: 49031

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