Set output path for bc files

- Updates spm to pass the object file path as the output path for
  bitcode files for use when building with lto. A more ideal solution
  may be to use a different path for bitcode files, however using the
  `.o` extension is also a common pattern and requires a significantly
  less invasive diff.

Author: rauhul

Sources/Build/BuildDescription/SwiftTargetBuildDescription.swift

@@ -759,6 +759,7 @@ public final class SwiftTargetBuildDescription {
 
             content +=
                 #"""
+                    "llvm-bc": "\#(object._nativePathString(escaped: true))",
                     "object": "\#(object._nativePathString(escaped: true))",
                     "swiftmodule": "\#(partialModulePath._nativePathString(escaped: true))",
                     "swift-dependencies": "\#(swiftDepsPath._nativePathString(escaped: true))"