Empty generic parameter clause produces no diagnostic

[SimplyDanny]

Issue Kind

Bad Diagnostic Produced

Source Code

// Can be parsed and produces no diagnostics.
func f< >() {}

// Can be parsed. Printing the node or asking for the diagnostics leads to a crash.
func f<>() {} 

Description

In both examples, the "old" parser reports "Expected an identifier to name generic parameter" on > which is helpful, but not the real issue.

Best would be a message like "Generic parameter clause cannot be empty" or similar. It should be reported in both examples. The ParseDiagnosticsGenerator should not crash in the second example.

[ahoppen]

rdar://107514649