[llvm-ar][mri] Ensure CREATE commands overwrite the output file

The CREATE/CREATETHIN commands should overwrite the output file:
https://sourceware.org/binutils/docs/binutils/ar-scripts.html.

This fixes a regression for MRI scripts introduced in:
https://reviews.llvm.org/D123142 which put logic into
performWriteOperation. performWriteOperation is called for all MRI
commands that write an archive out (one's with a SAVE command).
performWriteOperation is unaware of MRI semantics and loads an
existing archive if present. If an existing archive is loaded, llvm-ar
checks the properties of the existing archive for decisions about the
output archive (for example making the output archive thin if the
existing one was). https://reviews.llvm.org/D123142 adds the following
logic...

if (OldArchive) {
  if (Thin && !OldArchive->isThin())
    fail("cannot convert a regular archive to a thin one");
  if (OldArchive->isThin())
    Thin = true;
}

... which errors for a script with CREATETHIN in effect if there is an
existing regular archive, and causes CREATE to output a thin archive
if there is an existing thin archive.

Differential Revision: https://reviews.llvm.org/D125439

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