Properly track l-paren of a CXXFucntionalCastExpr.

In addition to storing more useful information in the AST, this
fixes a semantic check in template instantiation which checks whether
the l-paren location is valid.

Fixes PR16903.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188495 91177308-0d34-0410-b5e6-96231b3b80d8