Minor clean up in llvm detection

When calling llvm-config to determine the installation prefix, ignore
output on stderr (possibly from the shell), to avoid confusing output
when llvm-config is not even installed or in the PATH.

Change-Id: I9534ffe90c20040972f3b5390f58936ac4839670
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Martin Smith <martin.smith@qt.io>

mkspecs/features/qt_find_clang.prf

@@ -57,7 +57,7 @@ for(_, $$list(_)) { # just a way to break easily
     }
     isEmpty(LLVM_INSTALL_DIR): LLVM_INSTALL_DIR = $$(LLVM_INSTALL_DIR)
     # Assume libclang is installed on the target system
-    isEmpty(LLVM_INSTALL_DIR): LLVM_INSTALL_DIR = $$system("llvm-config --prefix")
+    isEmpty(LLVM_INSTALL_DIR): LLVM_INSTALL_DIR = $$system("llvm-config --prefix 2>$$QMAKE_SYSTEM_NULL_DEVICE")
     LLVM_INSTALL_DIR = $$clean_path($$LLVM_INSTALL_DIR)
 
     contains(QMAKE_HOST.arch, x86_64): \