Add test for old issue w/ bad source locations for warnings in .lhs files (#515)

testsuite/tests/parser/should_compile/T515.lhs

+In #515, an LHS file gave warnings attributed to the wrong source location
+(i.e., line 1).  This regression tests makes sure they are attributed to
+the correct line.
+
+> module Test where
+> a = 1
+> b = 2
+
+The above should attribute the warnings to lines 6 and 7.

testsuite/tests/parser/should_compile/T515.stderr

+
+T515.lhs:6:3: warning: [-Wmissing-signatures (in -Wall)]
+    Top-level binding with no type signature: a :: Integer
+
+T515.lhs:6:7: warning: [-Wtype-defaults (in -Wall)]
+     Defaulting the following constraint to type ‘Integer’
+        Num p0 arising from the literal ‘1’
+     In the expression: 1
+      In an equation for ‘a’: a = 1
+
+T515.lhs:7:3: warning: [-Wmissing-signatures (in -Wall)]
+    Top-level binding with no type signature: b :: Integer
+
+T515.lhs:7:7: warning: [-Wtype-defaults (in -Wall)]
+     Defaulting the following constraint to type ‘Integer’
+        Num p0 arising from the literal ‘2’
+     In the expression: 2
+      In an equation for ‘b’: b = 2

testsuite/tests/parser/should_compile/all.T

@@ -144,3 +144,4 @@ test('T15781', normal, compile, [''])
 test('T16339', normal, compile, [''])
 test('T16619', requires_th, multimod_compile, ['T16619', '-v0'])
 test('T504', normal, compile, [''])
+test('T515', literate, compile, ['-Wall'])