[turbofan] Rename --context-specialization to --function-context-specialization.

[turbofan] Rename --context-specialization to --function-context-specialization.

This name makes it clear that the flag (also the variant in the Compiler)
is talking about specializing to the function context instead of i.e. the
native context.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1372513003

Cr-Commit-Position: refs/heads/master@{#30934}