Clang: Move clang paths helper functions to Core plugin

These functions require core plugin so they can't go
to utils library. At the same time to use them in
ProjectExplorer plugin there are not too many choices
where to put them without introducing new dependencies.

Change-Id: I3cccccffaae8ac4bbce924fd809b5423da5dc503
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/plugins/coreplugin/icore.h

@@ -97,6 +97,9 @@ public:
     static QString installerResourcePath();
     static QString documentationPath();
     static QString libexecPath();
+    static QString clangExecutable(const QString &clangBinDirectory);
+    static QString clangIncludeDirectory(const QString &clangVersion,
+                                         const QString &clangResourceDirectory);
 
     static QString versionString();
     static QString buildCompatibilityString();