Use consistent clang through build
The just-built Swift driver was picking up the system clang, which would
try to use the bfd linker instead of gold or lld. bfd doesn't like how
Swift handles protected symbols and fails with the following error:
```
error: link command failed with exit code 1 (use -v to see invocation)
/usr/bin/ld: /home/build-user/build/buildbot_linux/libdispatch-linux-x86_64/src/swift/CMakeFiles/swiftDispatch.dir/Block.swift.o: relocation R_X86_64_PC32 against protected symbol `$s8Dispatch0A13WorkItemFlagsVSYAAMc' can not be used when making a shared object
/usr/bin/ld: final link failed: bad value
```
This patch tells the Swift-Driver where it should find its clang, and
makes it match the clang that is in use through the rest of the build.
