[clang] Resolve FIXME: Look at E, not M (PR #85541)

[John McCall via cfe-commits]

rjmccall wrote:

Hmm.  The best solution there is probably to use a consistent representation but introduce some sort of `GLValueThatTheStandardAbsurdlyPretendsIsAnRValue` (definitely the only and best word for this, ship it) that we can use as the value category.  IIRC, something similar happens in C where the standard pretends that function l-values don't exist.

https://github.com/llvm/llvm-project/pull/85541