[Diagnostics] Fix compiler failure when report mode disabled

[kLabz]

Started with a workaround, but ended up finding the real issue so reverted the workaround; keeping it in history just in case it's needed again later on.

This fixes an "old" (from january) issue that I could only get a clean repro for today:

File "src/core/globals.ml", line 175, characters 1-7: Assertion failed
Compiler failure
Please submit an issue at https://github.com/HaxeFoundation/haxe/issues/new
Attach the following information:
Haxe: 5.0.0-alpha.1+15731ea67; OS type: windows;
File "src/compiler/compiler.ml", line 26, characters 17-24
Called from file "src/compiler/compiler.ml", line 298, characters 2-275
Called from file "src/compiler/compiler.ml", line 365, characters 37-100
Called from file "src/compiler/compiler.ml", line 394, characters 1-5
Called from file "src/compiler/compiler.ml", line 471, characters 2-135
Called from file "src/compiler/compiler.ml", line 445, characters 2-9
Called from file "src/compiler/compiler.ml", line 668, characters 5-43
Called from file "src/compiler/compiler.ml", line 680, characters 13-22
Called from file "src/compiler/server.ml", line 633, characters 1-39
Called from file "src/compiler/server.ml", line 694, characters 3-30
Called from file "src/compiler/compiler.ml", line 668, characters 5-43
Called from file "src/compiler/compiler.ml", line 680, characters 13-22
Called from file "src/compiler/server.ml", line 633, characters 1-39
Called from file "src/compiler/haxe.ml", line 53, characters 0-56