Base optimization-record file names on the final output

Using Output.getFilename() to construct the file name used for optimization
recording in Clang::ConstructJob, when -c is provided, does not work correctly
if we're not using the integrated assembler. With -no-integrated-as (or
-save-temps) Output.getFilename() gives the name of the temporary assembly
file, not the final output file. Instead, use the final output (as provided by
-o). If this is not available, then fall back to using a name based on the
input file.

Fixes PR31532.

llvm-svn: 311041