[libclang] Fix the type of 'int (Foo);'

libclang exposes the type of 'int (Foo);' (a global variable of type int
called Foo) as CXType_Unexposed. This is because Clang represents Foo's
type as ParenType{BuiltinType{Int}}, and libclang does not handle
ParenType.

Make libclang return CXType_Int as the type of 'int (Foo);' by
unwrapping ParenType transparently.

Patch by Matt Glazar.

Differential Revision: https://reviews.llvm.org/D45713

llvm-svn: 331306