Rename functions which only print a diagnostic to Diagnose* (#3886)

This is something I noticed working on
https://github.com/carbon-language/carbon-lang/pull/3884; I think we
have more functions named Diagnose* at present than Emit* or Report*, so
just trying to consolidate. Note a couple Diagnose* functions do a
little more validation, but maybe those should actually be renamed away
(zygoloid had mentioned wanting to generally split out diagnostics to
their own function, and then we'd have it be a more common pattern).

toolchain/parse/handle_function.cpp

@@ -64,7 +64,7 @@ auto HandleFunctionSignatureFinish(Context& context) -> void {
       }
       auto semi = context.ConsumeIf(Lex::TokenKind::Semi);
       if (!semi && !state.has_error) {
-        context.EmitExpectedDeclSemi(context.tokens().GetKind(state.token));
+        context.DiagnoseExpectedDeclSemi(context.tokens().GetKind(state.token));
         state.has_error = true;
       }
       if (state.has_error) {
@@ -78,7 +78,7 @@ auto HandleFunctionSignatureFinish(Context& context) -> void {
     }
     default: {
       if (!state.has_error) {
-        context.EmitExpectedDeclSemiOrDefinition(Lex::TokenKind::Fn);
+        context.DiagnoseExpectedDeclSemiOrDefinition(Lex::TokenKind::Fn);
       }
       // Only need to skip if we've not already found a new line.
       bool skip_past_likely_end =