Use "foo-12345.o" instead of "foo.o-12345" as temporary file name.

This helps some tools that do things based on the output's extension.

For example, we got reports from users on Windows that have a tool that scan a
build output dir (but skip .obj files). The tool would keep the "foo.obj-12345"
file open, and then when clang tried to rename the temp file to the final
output filename, that would fail. By making the tempfile end in ".obj", tools
like this will now skip the temp files as well.

https://reviews.llvm.org/D36238

llvm-svn: 309984