Debug Info! Add basic support for line tables, compilation units, files,

and lexical scopes, which can be enabled through the new -g option.
When -g is enabled, line tables and scopes compile all the way
down to DWARF.

Changes to SIL:
- In addition to a SILLocation, every instruction now also has a pointer
  to a SILDebugScope (its containing lexical scope).
- Added LexicalScope, which is to be used for all Scopes we want to show
  up in the debug info.

Swift SVN r5772

test/DebugInfo/basic.swift

@@ -0,0 +1,28 @@
+// A very basic test for debug info.
+
+// Verify that we don't emit any debug info by default.
+// RUN: %swift -triple x86_64-apple-darwin10 %s -emit-llvm -o - | FileCheck %s --check-prefix NDEBUG
+// NDEBUG-NOT: !dbg
+// NDEBUG-NOT: DW_TAG
+
+// Now check that we do generate line+scope info with -g.
+// RUN: %swift -triple x86_64-apple-darwin10 %s -emit-llvm -g -o - | FileCheck %s
+
+func f(a: Int, b:Int) -> Int {
+     // CHECK-DAG: i32 [[@LINE-1]],{{.*}}DW_TAG_lexical_block
+     if b != 0 {
+       // CHECK-DAG: i32 [[@LINE-1]],{{.*}}DW_TAG_lexical_block
+       // CHECK-DAG: ret{{.*}}, !dbg ![[DBG:[0-9]+]]
+       // CHECK-DAG: [[DBG]] = metadata !{i32 [[@LINE+1]],
+       return a/b
+     } else {
+       // CHECK-DAG: [[PARENT:[0-9]+]] = {{.*}}i32 [[@LINE-1]],{{.*}}DW_TAG_lexical_block
+       var c = 42
+       if a == 0 {
+         // CHECK-DAG: metadata ![[PARENT]], i32 [[@LINE-1]],{{.*}}DW_TAG_lexical_block
+         // What about a nested scope?
+         return 0
+       }
+       return c
+     }
+}