This commit is the result of a general audit of

the expression parser to locate instances where
dyn_cast<>() and isa<>() are used on types, and
replace them with getAs<>() as appropriate.

The difference is that dyn_cast<>() and isa<>()
are essentially LLVM/Clang's equivalent of RTTI
-- that is, they try to downcast the object and
return NULL if they cannot -- but getAs<>() can
traverse typedefs to perform a semantic cast.

llvm-svn: 146537