rustc: Tweak the `isExported` callback for ThinLTO
rustc: Tweak the `isExported` callback for ThinLTO

Previously we were too eagerly exporting almost all symbols used in ThinLTO
which can cause a whole host of problems downstream! This commit instead fixes
this error by aligning more closely with `lib/LTO/LTO.cpp` in LLVM's codebase
which is to only change the linkage of summaries which are computed as dead.

Closes #46374

aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust