Split getOpenFile into getOpenFile and getOpenFileSlice.

The main observation is that we never need both the filesize and the map size.
When mapping a slice of a file, it doesn't make sense to request a null
terminator and that would be the only case where the filesize would be used.

There are other cleanups that should be done in this area:

* A client should not have to pass the size (even an explicit -1) to say if
  it wants a null terminator or not, so we should probably swap the argument
  order.
* The default should be to not require a null terminator. Very few clients
  require this, but many end up asking for it just because it is the default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186984 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/Support/MemoryBufferTest.cpp

@@ -113,13 +113,10 @@ TEST_F(MemoryBufferTest, getOpenFileNoNullTerminator) {
   }
 
   OwningBuffer Buf;
-  error_code EC = MemoryBuffer::getOpenFile(TestFD,
-                                            TestPath.c_str(),
-                                            Buf,
-                                            40000,    // Size
-                                            -1,
-                                            8000,     // Offset
-                                            false);
+  error_code EC = MemoryBuffer::getOpenFileSlice(TestFD, TestPath.c_str(), Buf,
+                                                 40000, // Size
+                                                 8000   // Offset
+                                                 );
   EXPECT_FALSE(EC);
 
   StringRef BufData = Buf->getBuffer();