Optimized the way breakpoint conditions are evaluated.
Previously, the options for a breakopint or its
locations stored only the text of the breakpoint
condition (ironically, they used ClangUserExpression
as a glorified std::string) and, each time the condition
had to be evaluated in the StopInfo code, the expression
parser would be invoked via a static method to parse and
then execute the expression.

I made several changes here:

  - Each breakpoint location now has its own
    ClangUserExpressionSP containing a version of
    the breakpoint expression compiled for that exact
    location.

  - Whenever the breakpoint is hit, the breakpoint
    condition expression is simply re-run to determine
    whether to stop.

  - If the process changes (e.g., it's re-run) or
    the source code of the expression changes (we use
    a hash so as to avoid doing string comparisons)
    the ClangUserExpressionSP is re-generated.

This should improve performance of breakpoint
conditions significantly, and takes advantage of
the recent expression re-use work.