[Support] Move llvm::MemoryBuffer to sys::fs::file_t

Summary:
On Windows, Posix integer file descriptors are a compatibility layer
over native file handles provided by the C runtime. There is a hard
limit on the maximum number of file descriptors that a process can open,
and the limit is 8192. LLD typically doesn't run into this limit because
it opens input files, maps them into memory, and then immediately closes
the file descriptor. This prevents it from running out of FDs.

For various reasons, I'd like to open handles to every input file and
keep them open during linking. That requires migrating MemoryBuffer over
to taking open native file handles instead of integer FDs.

Reviewers: aganea, Bigcheese

Reviewed By: aganea

Subscribers: smeenai, silvas, mehdi_amini, hiraditya, steven_wu, dexonsmith, dang, llvm-commits, zturner

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63453

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365588 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/FileOutputBuffer.cpp

@@ -147,7 +147,8 @@ createOnDiskBuffer(StringRef Path, size_t Size, unsigned Mode) {
   // Mmap it.
   std::error_code EC;
   auto MappedFile = llvm::make_unique<fs::mapped_file_region>(
-      File.FD, fs::mapped_file_region::readwrite, Size, 0, EC);
+      fs::convertFDToNativeFile(File.FD), fs::mapped_file_region::readwrite,
+      Size, 0, EC);
 
   // mmap(2) can fail if the underlying filesystem does not support it.
   // If that happens, we fall back to in-memory buffer as the last resort.