[6.0] Add `--enable-code-coverage` to `swift build` (#7565)
Explanation: Add `--enable-code-coverage` to `swift build`. This flag
allows enabling code coverage when splitting a test run between `swift
build` and `swift test --skip-build`.
Scope: New option for the `swift build` command.
Original PR: #7508 (partial), #7518
Risk: Low. Behaviour is identical for `swift build` without the new
flag, and the new flag must be explicitly specified.
Testing: Ran `swift build` unit tests at desk and in CI.
Reviewer: @bnbarham, @MaxDesiatov, @stmontgomery
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Co-authored-by: Max Desiatov <m_desiatov@apple.com>