Emit .gcda files as absolute paths instead of relative paths.

This improves compatibility with gcc in this regard, and this file generation
can be ameliorated with GCOV_PREFIX and GCOV_PREFIX_STRIP. It's also useful if
your build directory doesn't specify -o <abspath> and it uses a recursive make
structure, so it's not relative to the toplevel.

Patch by Joshua Cranmer!
<rdar://problem/12179524>

llvm-svn: 162884