subdirectories mirroring where the test case file is located

For example, for the test case "Sema/stmt_exprs.c", instead of the files
"Output/stmt_exprs.c.out" and "Output/stmt_exprs.c.out.script" being created, the
files "Output/Sema/stmt_exprs.c.out" and "Output/Sema/stmt_exprs.c.out.script" are
created. This prevents any collisions from different test directories that have the
same file name for a test case, and also makes it clear where the test case was
drawn from.

llvm-svn: 44410