[clang] [llvm] [llvm, clang] Don't assume non-erased DenseMap entries remain valid after erase. NFC (PR #198982)

[Fangrui Song via cfe-commits]

MaskRay wrote:

Confirmed

`Two callers ⇒ the compiler emits DenseMapBase::remove_if<$_0> out-of-line rather than inlining it.`

Created a fix: https://github.com/llvm/llvm-project/pull/199571

https://github.com/llvm/llvm-project/pull/198982