Fixed a bunch of ownership problems with the expression

parser.  Specifically:

- ClangUserExpression now keeps weak pointers to the
  structures it needs and then locks them when needed.
  This ensures that they continue to be valid without
  leaking memory if the ClangUserExpression is long
  lived.

- ClangExpressionDeclMap, instead of keeping a pointer
  to an ExecutionContext, now contains an
  ExecutionContext.  This prevents bugs if the pointer
  or its contents somehow become stale.  It also no
  longer requires that ExecutionContexts be passed
  into any function except its initialization function,
  since it can count on the ExecutionContext still
  being around.

There's a lot of room for improvement (specifically,
ClangExpressionDeclMap should also use weak pointers
insetad of shared pointers) but this is an important
first step that codifies assumptions that already
existed in the code.

llvm-svn: 150217