build: indicate VC++ CRT type for Windows builds

[compnerd]

Indicate whether the debug or release CRT is in use when building for
Windows.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[troughton]

Would it be better for the debug CRT to be conditional on DEBUG, since there are multiple types of release builds?

As a side note: is there actually a benefit to using the debug CRT? It doesn’t seem particularly useful for Swift binaries/libraries, and it’s a little odd that the compiler’s build configuration (or technically the stdlib) alters the produced Swift binaries.

[compnerd]

It actually is useful, the debug CRT is able to actually do additional debugging (like malloc tracing). This is just displaying the value. I don't have a problem with changing the condition, but I think that is better done in a follow up.

[compnerd]

@swift-ci please test and merge

[troughton]

Is there a way to implement it so that the configuration of the built Swift product (e.g. with swiftc -O for release) determines which CRT is used? Is it possible to leave the CRT symbols unresolved for the stdlib and then link against it when linking the final Swift executable?

As an aside, I still think RELEASE is the wrong condition here. I could go either way on RelWithDebInfo builds, but I definitely think MinSizeRel should use the non-debug CRT.