Auto merge of #145848 - Kobzol:optimize-file-read, r=the8472

Slightly optimize reading of source files

Discussed on Zulip (https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/rustc.20uses.20silly.20amount.20of.20syscalls.20for.20file.20IO/with/536015625.

Do not open/close each source file twice when reading it. We still stat it twice though.

Author: bors