...and then because of a compiler bug (SR-806), rename the helper
properties to 'asNative' and 'asCocoa'.
None of this is API, so there is no migration information.
This commit is the result of auditing the following files:
* StringBuffer.swift
* StringCharacterView.swift
* StringCore.swift
* StringInterpolation.swift.gyb
* StringLegacy.swift
Stack trace:
```
13 swift 0x0000000000f2f1a8 swift::Decl::dump(llvm::raw_ostream&, unsigned int) const + 152
18 swift 0x0000000000f2f1a8 swift::Decl::dump(llvm::raw_ostream&, unsigned int) const + 152
27 swift 0x0000000000f6d024 swift::Decl::walk(swift::ASTWalker&) + 20
28 swift 0x0000000001000a6e swift::SourceFile::walk(swift::ASTWalker&) + 174
29 swift 0x0000000000f53324 swift::verify(swift::SourceFile&) + 52
30 swift 0x0000000000dcd4e4 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int) + 1492
31 swift 0x0000000000c8023f swift::CompilerInstance::performSema() + 2975
33 swift 0x0000000000774691 frontend_main(llvm::ArrayRef<char const*>, char const*, void*) + 2481
34 swift 0x000000000076f1d5 main + 2773
Stack dump:
0. Program arguments: /path/to/build/Ninja-ReleaseAssert/swift-linux-x86_64/bin/swift -frontend -c -primary-file validation-test/compiler_crashers/28255-swift-decl-dump.swift -target x86_64-unknown-linux-gnu -disable-objc-interop -module-name main -o /tmp/28255-swift-decl-dump-ac2682.o
1. While walking into decl 'Q' at validation-test/compiler_crashers/28255-swift-decl-dump.swift:8:1
2. While walking into body of 'Q' at validation-test/compiler_crashers/28255-swift-decl-dump.swift:8:1
3. While verifying overridden 'init' at validation-test/compiler_crashers/28255-swift-decl-dump.swift:8:23
<unknown>:0: error: unable to execute command: Aborted
<unknown>:0: error: compile command failed due to signal (use -v to see invocation)
```
`FixedPoint.swift.gyb` generates a massive amount of assertions:
one for each `Int` type (such as `UInt8` and `Int64`) as a source,
to one of each `Int` type as a destination, for each of 27 bit
patterns. By my math that's 8 * 8 * 27 == 1728 assertions.
As a result, when one of those assertions fails, it's difficult to
tell what went wrong.
Split each assertion into its own test case. This makes the test
take a little longer to run, but it produces much more valuable
output when it fails.
