🐣 GN: Print the import trail when parse errors occur.

tools/gn/location.cc

Index: tools/gn/location.cc
diff --git a/tools/gn/location.cc b/tools/gn/location.cc
index 49ca3ffe717f7c1550eae1f25f4c28f73b63b6ac..ce4f06ff391608438ae87262f9af5cd4f97aa2f2 100644
--- a/tools/gn/location.cc
+++ b/tools/gn/location.cc
@@ -26,6 +26,10 @@ Location::Location(const InputFile* file,
       byte_(byte) {
 }
 
+bool Location::is_null() const {
+  return *this == Location();
+}
+
 bool Location::operator==(const Location& other) const {
   return other.file_ == file_ &&
          other.line_number_ == line_number_ &&
@@ -70,6 +74,10 @@ LocationRange::LocationRange(const Location& begin, const Location& end)
   DCHECK(begin_.file() == end_.file());
 }
 
+bool LocationRange::is_null() const {
+  return begin_.is_null();  // No need to check both for the null case.
+}
+
 LocationRange LocationRange::Union(const LocationRange& other) const {
   DCHECK(begin_.file() == other.begin_.file());
   return LocationRange(