DebugInfo: Elide lexical scopes which only contain other (inline or lexical) scopes.

DW_TAG_lexical_scopes inform debuggers about the instruction range for
which a given variable (or imported declaration/module/etc) is valid. If
the scope doesn't itself contain any such entities, it's a waste of
space and should be omitted.

We were correctly doing this for entirely empty leaves, but not for
intermediate nodes.

Reduces total (not just debug sections) .o file size for a bootstrap
-gmlt LLVM by 22% and bootstrap -gmlt clang executable by 13%. The wins
for a full -g build will be less as a % (and in absolute terms), but
should still be substantial - with some of that win being fewer
relocations, thus more substantiall reducing link times than fewer bytes
alone would have.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216861 91177308-0d34-0410-b5e6-96231b3b80d8

test/DebugInfo/missing-abstract-variable.ll

@@ -45,8 +45,6 @@
 ; CHECK-NOT: DW_TAG
 ; CHECK:     DW_AT_name {{.*}} "b"
 ; CHECK-NOT: {{DW_TAG|NULL}}
-; CHECK:     DW_TAG_lexical_block
-; CHECK-NOT: {{DW_TAG|NULL}}
 ; CHECK:       DW_TAG_lexical_block
 ; CHECK-NOT: {{DW_TAG|NULL}}
 ; CHECK: [[ABS_S:.*]]:       DW_TAG_variable
@@ -89,19 +87,12 @@
 ; CHECK-NOT: DW_TAG
 ; CHECK:       DW_AT_abstract_origin {{.*}} {[[ABS_B]]}
 
-; The two lexical blocks here are caused by the scope of the if that includes
-; the condition variable, and the scope within the if's composite statement. I'm
-; not sure we really need both of them since there's no variable declared in the
-; outer of the two
-
 ; CHECK-NOT: {{DW_TAG|NULL}}
 ; CHECK:     DW_TAG_lexical_block
 ; CHECK-NOT: {{DW_TAG|NULL}}
-; CHECK:       DW_TAG_lexical_block
-; CHECK-NOT: {{DW_TAG|NULL}}
-; CHECK:         DW_TAG_variable
+; CHECK:       DW_TAG_variable
 ; CHECK-NOT: DW_TAG
-; CHECK:           DW_AT_abstract_origin {{.*}} {[[ABS_S]]}
+; CHECK:         DW_AT_abstract_origin {{.*}} {[[ABS_S]]}
 
 @t = external global i32