[clang][ToolChains] explicitly return LangOptions::StackProtectorMode

Make the virtual method Toolchain::GetDefaultStackProtectorLevel()
return an explict enum value rather than an integral constant. This
makes the code subjectively easier to read, and should help prevent bugs
that may (or may never) arise from changing the enum values. Previously,
these were just kept in sync via a comment, which is brittle. The trade
off is including a additional header in a few new places. It is not
necessary, but in my opinion helps the readability.

Split off from https://reviews.llvm.org/D90194 to help cut down on lines
changed in code review.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D90271

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