Fix qbs build when no llvm is present

Some defines were set conditionally, but accessed unconditionally, which
of course breaks the build.
Make sure the defines are always set. Also gather them in a module,
instead of copying the code all over the place.

Change-Id: Iac628da64fb3934872d912510936aeccd0e20247
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/clangrefactoring/clangrefactoring.qbs

@@ -5,6 +5,7 @@ QtcPlugin {
     name: "ClangRefactoring"
 
     Depends { name: "libclang"; required: false }
+    Depends { name: "clang_defines" }
     condition: libclang.present && libclang.toolingEnabled
 
     Depends { name: "ClangSupport" }
@@ -16,19 +17,7 @@ QtcPlugin {
     Depends { name: "ProjectExplorer" }
     Depends { name: "TextEditor" }
 
-    cpp.defines: {
-        var defines = base;
-        defines.push("CLANGPCHMANAGER_LIB");
-
-        // The following defines are used to determine the clang include path for intrinsics.
-        defines.push('CLANG_VERSION="' + libclang.llvmVersion + '"');
-        var resourceDir = FileInfo.joinPaths(libclang.llvmLibDir, "clang", libclang.llvmVersion,
-                                             "include");
-        defines.push('CLANG_RESOURCE_DIR="' + resourceDir + '"');
-        defines.push('CLANG_BINDIR="' + libclang.llvmBinDir + '"');
-        return defines;
-    }
-
+    cpp.defines: base.concat("CLANGPCHMANAGER_LIB")
     cpp.includePaths: ["."]
 
     files: [