Collect profiles for Clang

Bug: http://b/37574207

Add support in build.py for building an instrumented stage2.  Add flag
to test_compiler.py that builds Android host and target modules using an
instrumented Clang, and merges the collected profiles.

Test: toolchain/llvm/android/build.py --build-instrumented
      toolchain/llvm/android/test_compiler.py --build-only --target \
          aosp_marlin-eng ./ --generate-clang-profile

Change-Id: I1b33d1b5edb5cd53fda811c4cc4832ac2adfa1f4