SwiftCompilerSources: force the -static flag

These libraries are built statically for static linking.  SPM currently
does not support libraries, and this will ensure that we properly build
the library target for consumption into the compiler.

SwiftCompilerSources/Package.swift

@@ -29,6 +29,7 @@ private extension Target {
         swiftSettings: [
           .interoperabilityMode(.Cxx),
           .unsafeFlags([
+            "-static",
             "-Xcc", "-I../include",
             "-Xcc", "-I../../llvm-project/llvm/include",
             "-Xcc", "-I../../llvm-project/clang/include",