Bug #:

Submitted by:
Reviewed by:
Work on finishing up typechecking for simple assignments (=) and function
calls. Here is an overview:
- implemented type checking for function calls (in Sema::ParseCallExpr).
- refactored UsualAssignmentConversions to return the result of the conversion.
This enum will allow all clients to emit different diagnostics based on context.
- fixed bug in Expr::isLvalue()...it wasn't handling arrays properly. Also
changed the name to isModifiableLvalue, which is consistent with the function on QualType.
- Added 6 diagnostics (3 errors, 3 extensions).

llvm-svn: 39432