Cache compiler info in DynFlags

Signed-off-by: Austin Seipp <austin@well-typed.com>
Don't pass -nodefaultlibs to Clang

This fixes a large majority of the testsuite failures on Mavericks with
Clang.

Signed-off-by: Austin Seipp <austin@well-typed.com>
Fix ghciprog004 when built with Clang

Clang warns about implicit return values by default.

Signed-off-by: Austin Seipp <austin@well-typed.com>