QmlJS: Don't recalculate the empty library's fingerprint

LibraryInfo::calculateFingerprint() is expensive. The default ctor will
always result in the same fingerprint. We only need to calculate it
once.

Change-Id: I680dc63624d68ddeca7f3467f4518d767274b246
Reviewed-by: Marco Benelli <marco.benelli@qt.io>

src/libs/qmljs/qmljsdocument.cpp

@@ -354,6 +354,14 @@ Bind *Document::bind() const
     return _bind;
 }
 
+LibraryInfo::LibraryInfo()
+    : _status(NotScanned)
+    , _dumpStatus(NoTypeInfo)
+{
+    static const QByteArray emptyFingerprint = calculateFingerprint();
+    _fingerprint = emptyFingerprint;
+}
+
 LibraryInfo::LibraryInfo(Status status)
     : _status(status)
     , _dumpStatus(NoTypeInfo)

src/libs/qmljs/qmljsdocument.h

@@ -164,7 +164,8 @@ private:
     QString _dumpError;
 
 public:
-    explicit LibraryInfo(Status status = NotScanned);
+    LibraryInfo();
+    explicit LibraryInfo(Status status);
     explicit LibraryInfo(const QmlDirParser &parser, const QByteArray &fingerprint = QByteArray());
     ~LibraryInfo();