llvm-extract now supports unnamed basic blocks.

[AllinLeeYL]

This PR enables the llvm-extract tool to support unnamed basic blocks with only a few lines of modification on llvm/tools/llvm-extract/llvm-extract.cpp.

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[AllinLeeYL]

Dear developer:

I have recently working with LLVM IR and I want to isolate basic blocks using the command "llvm-extract". However, I found that the command option "llvm-extract --bb func_name:bb_name" will only function when dumping source code into IRs with options "-fno-discard-value-names". That is to say, the "llvm-extract" command cannot support unnamed basic blocks, which is a default output of the compiler. So, I made these changes and hope they will make LLVM better.

Best regards,