Make a general helper function on the AST context to retrieve a type by identifier in the fashion needed by data formatters

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@220059 91177308-0d34-0410-b5e6-96231b3b80d8
Fixed the expression parser to handle cases where
GetValueForVariableExpressionPath returns NULL and
doesn't set an error.

<rdar://problem/18682916>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@220070 91177308-0d34-0410-b5e6-96231b3b80d8