Fix CONFIG+=silent for MSVC non-inference rules

silent.prf modifies the compiler commands by prefixing them with a
silencing echo command. For MSVC, the used $< syntax is only valid in
inference rules. However, the PCH rule is not an inference
rule and breaks when silent.prf is used.

Remove the echo command for MSVC. The compiler already outputs the
currently compiled file. There's no need to do it twice.

Task-number: QTBUG-61688
Change-Id: I7e2c1211e471c9c149c16cac8e87406e88ee2d97
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

mkspecs/features/silent.prf

@@ -1,6 +1,11 @@
 !macx-xcode {
-    QMAKE_CC = @echo compiling $< && $$QMAKE_CC
-    QMAKE_CXX = @echo compiling $< && $$QMAKE_CXX
+    msvc {
+        QMAKE_CC = @$$QMAKE_CC
+        QMAKE_CXX = @$$QMAKE_CXX
+    } else {
+        QMAKE_CC = @echo compiling $< && $$QMAKE_CC
+        QMAKE_CXX = @echo compiling $< && $$QMAKE_CXX
+    }
     !contains(QMAKE_LINK, "@:"):QMAKE_LINK = @echo linking $@ && $$QMAKE_LINK
     QMAKE_LINK_SHLIB = @echo linking $@ && $$QMAKE_LINK_SHLIB
 }