Speeds up parsing of global declarations in cases where the warning

is switched of by about 0.8% (tested with int i<N>).

Additionally, this puts computing the diagnostic class into the hot
path more when parsing, in preparation for upcoming optimizations
in this area.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@169976 91177308-0d34-0410-b5e6-96231b3b80d8