Debugger: Add interface for expression evaluation.

SourceLanguage:
- Add a virtual to the language base class that requests expression
  evaluation.
Debugger: Add initial skeleton for a C-style expression evaluator.

- Adds a stripped down version of ExpressionParser from libshared,
  which will eventually be modified to support various other capabilities
  needed for debug expressions. Not yet used or complete.
Update icu_bootstrap

Our code wouldn't compile with the old one anymore (issuing warnings
when -Werror is enabled).