Clean up changes suggested by Douglas Gregor:

BuildObjCNumericLiteral() and BuildObjCBoxedExpr() now both using
PerformCopyInitialization() rather than PerformImplicitConversion(),
which suppresses errors.

In BuildObjCBoxedExpr(): no longer calling .getCanonicalType(),
ValueType->getAs() will remove the minimal amount of sugar.
Using ValueType->isBuiltinType() instead of isa<BuiltinType>(ValueType).

llvm-svn: 155949