Clean up dependency .d.$$$$ files for tests

Summary:
Mac-only tests were leaving .d.$$$$ dependency files left in the test tree. This happened
because:
1) "make clean" was invoked although no tests in the folder were run
2) The Makefile had main.d as a dependency, which meant it tried to compile the source file (to extract
dependencies).
3) The compile failed (does not compile on linux) and left behind a main.d.$$$$ temporary file.
4) "make clean" tried to clean up these temporary files, but the expansion $(wildcard *.d.[0-9]*)
was performed before the temporary file was created, so this file was not caught.

I fix this by giving all the temporary files deterministic names, which makes it easier to clean
them up afterwards. I also make the rules for generating the dependency files more robust and
less likely to leak files in the first place.

Reviewers: vharron, zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D7406

llvm-svn: 228285