tools/gn: Add out-of-line copy ctors for complex classes.

tools/gn/substitution_pattern.cc

Index: tools/gn/substitution_pattern.cc
diff --git a/tools/gn/substitution_pattern.cc b/tools/gn/substitution_pattern.cc
index dd703d72c96a8eafeb8c292b5a257f1ea61f8864..c8e548998440e4814fc11f359ce0c6c922aeecec 100644
--- a/tools/gn/substitution_pattern.cc
+++ b/tools/gn/substitution_pattern.cc
@@ -28,6 +28,9 @@ SubstitutionPattern::Subrange::~Subrange() {
 SubstitutionPattern::SubstitutionPattern() : origin_(nullptr) {
 }
 
+SubstitutionPattern::SubstitutionPattern(const SubstitutionPattern& other) =
+    default;
+
 SubstitutionPattern::~SubstitutionPattern() {
 }