Auto merge of #51883 - estebank:placement-suggestion, r=varkor

Suggest correct comparison against negative literal

When parsing as emplacement syntax (`x<-1`), suggest the correct syntax
for comparison against a negative value (`x< -1`).

Fix #45651.

diff --cc src/librustc_passes/ast_validation.rs
Simple merge

diff --cc src/libsyntax/ast.rs
Simple merge