Rollup merge of #59874 - michaelwoerister:pgo-updates-1, r=cramertj

Clean up handling of `-Z pgo-gen` commandline option.

This PR adapts the `-Z pgo-gen` flag to how Clang and GCC handle the corresponding `-fprofile-generate` flag. In particular, the flag now optionally takes a directory to place the profiling data in and allows to omit the argument (instead of having to pass an empty string).

diff --cc src/librustc_codegen_llvm/back/write.rs
Simple merge

diff --cc src/librustc_codegen_ssa/back/write.rs
Simple merge