[HLSL][SPIRV] Add CLI option `-fspv-extension` (#137985)
This commit implements DXC's `-fspv-extension` options. It is
implemented by replaced it with the equivalent `-spirv-ext` option. Note
that if the option is not used, that is the same as enabling all
extension, so `-spirv-ext=all` is used.
Fixes https://github.com/llvm/llvm-project/issues/137647
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Co-authored-by: Cassandra Beckley <cbeckley@google.com>
