Optimize moc: Preallocate space for list of preprocessed symbols.

Without this, the symbol list is frequently reallocated. The value
is a guestimate. It corresponds to a block of memory with the size

262144 * 3 * 8 byte = ca. 6.3 megabytes

on a 64 bit machine. Looking at the final size of the symbol list,
this seems to fit the common case nicely.

Change-Id: Ib546a1ea479f2c6d8ab57be783cdf630f9b54f77
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/tools/moc/preprocessor.cpp

@@ -1222,6 +1222,10 @@ Symbols Preprocessor::preprocessed(const QByteArray &filename, QFile *file)
 
     // phase 3: preprocess conditions and substitute macros
     Symbols result;
+    // Preallocate some space to speed up the code below.
+    // The magic value was found by logging the final size
+    // and calculating an average when running moc over FOSS projects.
+    result.reserve(file->size() / 300000);
     preprocess(filename, result);
     mergeStringLiterals(&result);