Remember if lseek works in this FD.

It will be used in clang in a sec.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234619 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/raw_ostream.h

@@ -333,6 +333,8 @@ class raw_fd_ostream : public raw_ostream {
 
   uint64_t pos;
 
+  bool SupportsSeeking;
+
   /// See raw_ostream::write_impl.
   void write_impl(const char *Ptr, size_t Size) override;
 
@@ -370,6 +372,8 @@ public:
   /// fsync.
   void close();
 
+  bool supportsSeeking() { return SupportsSeeking; }
+
   /// Flushes the stream and repositions the underlying file descriptor position
   /// to the offset specified from the beginning of the file.
   uint64_t seek(uint64_t off);