Parsing of member bounds declarations.

[dtarditi]

This change adds parsing of member bounds declarations (issue #19). Member bounds
declarations use member bounds expressions, which can only use other
members of the structure. The changes include:

• IR: no change is needed to represent member bounds expressions. Because
  clang handles C++, it already can represent members directly in
  the IR for expressions. Bounds expressions are a subclass of expressions.

  We need to attach bounds expressions to member declarations. We already
  have bounds expressions on function declarations and variable declarations,
  which have the same base class as member declarations. Instead of having
  a 3rd copy of the code, we factor the common code for representing bounds
  expressions and move it to the common base.

• Parsing: we modify the parsing of member declarators that are followed
  by ':' to parse either a bounds expression or a constant-expression
  that is a bit-field size. We check whether the first token after the
  ':' is an identifier that is contextual keyword for bounds expressions.

  The parsing of bounds expressions is deferred until all members of the
  structure have been seen. This is so that member bounds expressions may
  refer to any member of the structure. Constant expressions for bitfield
  sizes continue to be eagerly parsed.

  The decision to just look for a bounds expression keyword can lead
  to several error messages from clang if there is a spelling
  error in a contextual keyword. Clang interprets the misspelled keyword
  as an implicit function declaration and issues unknown identifier errors
  for uses of members.

  For now, we'll live this because this specific case is a fairly
  low priority to address. The type for the identifier in the declarator
  hasn't been built yet at the time that we make the parsing decision.
  We could move that up or defer parsing of constant expressions. Either
  change is largely orthogonal to this change.

• Semantic processing: member bounds expressions are restricted to accessing
  only members. We add a state flag to Sema that changes the lookup of C
  identifiers during expression parsing to look up identifiers in the
  member namespace instead of the ordinary namespace. The state flag is
  protected by an RAII class.

Testing:

• Wrote new feature tests of parsing member bounds
  declaration. These will be checked in separately to the Checked C
  repo in parsing\member_bounds.c.
• Passes existing clang test baseline.

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[reubeno]

ReturnBoundsExpr [](start = 14, length = 16)

I'm not sure where it would go, but it might be helpful to have a comment somewhere explain the intended meaning of 'Bounds' for FunctionDecls--that it's intended to describe the return value of the function. #Resolved

[dtarditi]

There are already comments on the getter functions in the superclass of FunctionDecl. These will be picked up the automatic doc generation and by IDE tool tips (for IDEs that understand /brief comments). I've add a comment to the setter function too. #Resolved

[reubeno]

This generally looks good to me--just had a few low-priority suggestions around the code and a few questions around the lifetime of cached tokens.