[llvm] [TableGen] Use a more efficient memory buffer for output (PR #123353)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

@jayfoad Would it be acceptable to call `std::string::reserve` with the size of the existing output file?  It might seem less elegant to rely on the file to be overwritten, but we read the existing output file for comparison purposes anyway, so we might as well use its size.

https://github.com/llvm/llvm-project/blob/69d0c4c1675c90cf99126210679a9c3ae0a8637e/llvm/lib/TableGen/Main.cpp#L155-L163


https://github.com/llvm/llvm-project/pull/123353