[COFF] create MemoryBuffers without requiring NUL terminators

[inglorion]

Summary

In a number of places in the COFF linker, we were calling
MemoryBuffer::getFile() with default parameters. This causes LLVM to
NUL-terminate the buffers, which can prevent them from being memory
mapped. Since we operate on binary and do not use NUL as an indicator
of the end of the file content, this change causes us to not require
the NUL terminator anymore.

[inglorion]

Anyone able to take a look at this?

[ruiu]

LGTM

COFF/Driver.cpp

 #if _WIN32
   // On Windows, file I/O is relatively slow so it is best to do this
   // asynchronously.
   auto Strategy = std::launch::async;
 #else
   auto Strategy = std::launch::deferred;
 #endif
   return std::async(Strategy, [=]() {
-    auto MBOrErr = MemoryBuffer::getFile(Path);
+    auto MBOrErr = MemoryBuffer::getFile(Path,
+                                         /*FileSize*/ -1,
+                                         /*RequiresNullTerminator*/ false);
     if (!MBOrErr)
       return MBErrPair{nullptr, MBOrErr.getError()};
     return MBErrPair{std::move(*MBOrErr), std::error_code()};
   });
 }
 
 MemoryBufferRef LinkerDriver::takeBuffer(std::unique_ptr<MemoryBuffer> MB) {
   MemoryBufferRef MBRef = *MB;
static void createImportLibrary(bool AsLib) {
   if (!Config->Incremental) {
     HandleError(writeImportLibrary(LibName, Path, Exports, Config->Machine,
                                    false, Config->MinGW));
     return;
   }
 
   // If the import library already exists, replace it only if the contents
   // have changed.
-  ErrorOr<std::unique_ptr<MemoryBuffer>> OldBuf = MemoryBuffer::getFile(Path);
+  ErrorOr<std::unique_ptr<MemoryBuffer>> OldBuf = MemoryBuffer::getFile(
+      Path, /*FileSize*/ -1, /*RequiresNullTerminator*/ false);
   if (!OldBuf) {
     HandleError(writeImportLibrary(LibName, Path, Exports, Config->Machine,
                                    false, Config->MinGW));
     return;
   }
 
   SmallString<128> TmpName;
   if (std::error_code EC =
           sys::fs::createUniqueFile(Path + ".tmp-%%%%%%%%.lib", TmpName))
     fatal("cannot create temporary file for import library " + Path + ": " +
           EC.message());
 
   if (Error E = writeImportLibrary(LibName, TmpName, Exports, Config->Machine,
                                    false, Config->MinGW)) {
     HandleError(std::move(E));
     return;
   }
 
-  std::unique_ptr<MemoryBuffer> NewBuf = check(MemoryBuffer::getFile(TmpName));
+  std::unique_ptr<MemoryBuffer> NewBuf = check(MemoryBuffer::getFile(
+      TmpName, /*FileSize*/ -1, /*RequiresNullTerminator*/ false));
   if ((*OldBuf)->getBuffer() != NewBuf->getBuffer()) {
     OldBuf->reset();
     HandleError(errorCodeToError(sys::fs::rename(TmpName, Path)));
   } else {
     sys::fs::remove(TmpName);
   }
 }