Fix broken implicit import conditional rename.

In #37686 `SWIFT_IMPLICIT_CONCURRENCY_IMPORT` was introduced alongside
the existing `SWIFT_ENABLE_EXPERIMENTAL_CONCURRENCY`, but the
`#cmakedefine` for this preprocessor symbol was renamed to the former
instead of a new `#cmakedefine` being introduced.

This means the `#ifdef` conditional referred in this commit was broken.
Instead of just introducing the old symbol back again, let's rename it
as it probably was intended here.

However, the semantics of this symbol is slightly different: this now
requires prospective builders to explicitly turn this off on platforms
where Dispatch is unavailable. To address this, the docs are updated to
match. Caveat: I have not ran the procedure in the doc recently, but
this hopefully should suffice to guide the reader until I get around to
doing so.

Author: 3405691582

docs/OpenBSD.md

@@ -82,6 +82,7 @@ $ ./utils/build-script \
         -DSWIFT_BUILD_SOURCEKIT=OFF,\
         -DSWIFT_BUILD_SYNTAXPARSERLIB=OFF,\
         -DSWIFT_ENABLE_EXPERIMENTAL_CONCURRENCY=OFF,\
+        -DSWIFT_IMPLICIT_CONCURRENCY_IMPORT=OFF,\
         -DSWIFT_USE_LINKER=lld,\
         -DCMAKE_INSTALL_DIR=/usr/local"
 ```

lib/Frontend/Frontend.cpp

@@ -767,7 +767,7 @@ static bool shouldImportConcurrencyByDefault(const llvm::Triple &target) {
     return true;
   if (target.isOSLinux())
     return true;
-#if SWIFT_ENABLE_EXPERIMENTAL_CONCURRENCY
+#if SWIFT_IMPLICIT_CONCURRENCY_IMPORT
   if (target.isOSOpenBSD())
     return true;
 #endif