We have been using the default names for global symbols to this point.
This change introduces proper name mangling for functions.
This requires introducing a CXXABI class in the CIRGenModule. Because
only target independent name mangling is handled in this patch, the
CXXABI class does not require a target-specific implementation. The
general mechanism for selecting an implementation is introduced here,
but the actual target-specific subclasses are deferred until needed.
Lowering of int-to-bool casts had been left as NYI because the incubator
implemented it by lowering to cir.cmp, which hasn't been upstreamed yet,
but there is no reason this cast can't be lowered directly to LLVM's
compare operation when we're lowering directly to the LLVM dialect.
This change lowers the cast directly to an LLVM compare to zero.
