Rustc does not free memory before calling the linker

[cjgillot]

Hello,

While compiling rust-analyzer, I witnessed elevated memory usage by rustc while the linker is active. In that case, more that 600 MB is allocated by the compiler. In cargo-parallelized build, this is a lot.

rustc version: 1.40.0-nightly (5da4b4d47 2019-10-28)

Thank you for investigating this.

[matprec]

I think this can closed now that #66707 is merged.