Clang: Fix MSVC build

Link error:
clangpchmanagerbackendmain.obj : error LNK2019: unresolved external symbol
    "__declspec(dllimport) public: __cdecl ClangBackEnd::ClangPathWatcher<class QFileSystemWatcher,class QTimer>::ClangPathWatcher<class QFileSystemWatcher,class QTimer>(
        class ClangBackEnd::FilePathCachingInterface &,class ClangBackEnd::ClangPathWatcherNotifier *)"
    (__imp_??0?$ClangPathWatcher@VQFileSystemWatcher@@VQTimer@@@ClangBackEnd@@QEAA@AEAVFilePathCachingInterface@1@PEAVClangPathWatcherNotifier@1@@Z) referenced in function main

Looks like MSVC looks for import even for inline template functions.

Another alternative is to revert 351f355b69
and either ignore or suppress the GCC warning.

Change-Id: I040d4e3924af7f4d1f4424276329c6095e6ff6bd
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/libs/clangsupport/clangpathwatcher.h

@@ -77,7 +77,7 @@ using IdCache = StringCache<Utils::SmallString,
 
 template <typename FileSystemWatcher,
           typename Timer>
-class CLANGSUPPORT_EXPORT ClangPathWatcher : public ClangPathWatcherInterface
+class CLANGSUPPORT_GCCEXPORT ClangPathWatcher : public ClangPathWatcherInterface
 {
 public:
     ClangPathWatcher(FilePathCachingInterface &pathCache,

src/libs/clangsupport/clangsupport_global.h

@@ -39,6 +39,12 @@
 #  define CLANGSUPPORT_EXPORT Q_DECL_IMPORT
 #endif
 
+#ifdef Q_CC_GNU
+#  define CLANGSUPPORT_GCCEXPORT __attribute__((visibility("default")))
+#else
+#  define CLANGSUPPORT_GCCEXPORT
+#endif
+
 #ifndef CLANGBACKENDPROCESSPATH
 # define CLANGBACKENDPROCESSPATH ""
 #endif

src/libs/clangsupport/filepathcache.h

@@ -36,7 +36,7 @@
 namespace ClangBackEnd {
 
 template <typename FilePathStorage>
-class CLANGSUPPORT_EXPORT FilePathCache
+class CLANGSUPPORT_GCCEXPORT FilePathCache
 {
     using DirectoryPathCache = StringCache<Utils::PathString,
                                            int,