Satisfy g++ and adhere to a C++03 syntax quirk

Apparently, until C++11 you could not use global namespace qualifier for
a class name:
https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#355

Only g++ ever complained about it, and it seems to still do, so let's
just use a less precise qualification

Author: necto

clang/include/clang/StaticAnalyzer/Core/PathSensitive/SymbolManager.h

@@ -717,7 +717,7 @@ class SymbolVisitor {
 // allocated. This is usually stable across runs leading to the stability of
 // ConstraintMap and other containers using SymbolRef as keys.
 template <>
-struct ::llvm::ImutContainerInfo<clang::ento::SymbolRef>
+struct llvm::ImutContainerInfo<clang::ento::SymbolRef>
     : public ImutProfileInfo<clang::ento::SymbolRef> {
   using value_type = clang::ento::SymbolRef;
   using value_type_ref = clang::ento::SymbolRef;