Itanium Mangling: Fix handling of <expr-primary> in <template-arg>.

Previously, we were emitting an extraneous X .. E in <template-arg>
around an <expr-primary> if the template argument was constructed from
an expression (rather than an already-evaluated literal value).  In
such a case, we would then e.g. emit 'XLi0EE' instead of 'Li0E'.

We had one special-case for DeclRefExpr expressions, in particular, to
omit them the mangled-name without the surrounding X/E. However,
unfortunately, that special case also triggered for ParmVarDecl (a
subtype of VarDecl), and _incorrectly_ emitted 'L_Z .. E' instead of
the proper 'Xfp_E'.

This change causes mangleExpression itself to be responsible for
emitting X/E around non-primary expressions, which removes the
special-case, and corrects both these problems.

Differential Revision: https://reviews.llvm.org/D95487