Start adding the infrastructure for handling TypoExprs.

Part of the infrastructure is a map from a TypoExpr to the Sema-specific
state needed to correct it, along with helpers to ease dealing with the
state.

The the typo count is propagated up the stack of
ExpressionEvaluationContextRecords when one is popped off of to
avoid accidentally dropping TypoExprs on the floor. For example,
the attempted correction of g() in test/CXX/class/class.mem/p5-0x.cpp
happens with an ExpressionEvaluationContextRecord that is popped off
the stack prior to ActOnFinishFullExpr being called and the tree
transform for TypoExprs being run.

llvm-svn: 220695