Avoid invoking swiftc -front-end when building Dispatch overlay
Avoid invoking swiftc -front-end when building Dispatch overlay

Simplify the rules for compiling .swift source files when
building the wrapping overlay to avoid invoking the swiftc
front-end directly.  This avoids using an unsupported
swift compiler command line API and does the compilation
in a single shot with whole-module-optimization (potential
for better generated code).

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch