Fixing SourcePosition ambiguity

There is both a v8::internal::SourcePosition and a
v8::internal::torque::SourcePositon and in jumbo builds
an unqualified SourcePositon ended up referring to the wrong
one since nobody had told the compiler that the correct one
existed. This broke jumbo builds of v8 cctests
on Windows (because only in Windows will the compiler look for
the symbol in a parent namespace).

R=szuend@chromium.org

Bug: v8:8880
Change-Id: I7c9ebf68629642b65e86d6a8ae458ec5ff01f2ce
Reviewed-on: https://chromium-review.googlesource.com/c/1496972
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#59980}

src/torque/source-positions.h

@@ -13,6 +13,8 @@ namespace v8 {
 namespace internal {
 namespace torque {
 
+struct SourcePosition;
+
 class SourceId {
  public:
   static SourceId Invalid() { return SourceId(-1); }