Importer refactor

[milseman]

Finally separates out importFullName from the Impl, and it now only
relies on relevant Swift and Clang information, but no importer
state. Convenience functionality, better APIs, and more clean up
coming soon.

Pull omitNeedlessWordsInFunctionName off of the Impl, and into a
static function local to ImportName.cpp. Separate it out from the Impl
entirely. Though this adds a couple of extra ugly parameters, it's one
of the last bits of tie-in between importFullName and the Impl.

By refactoring out PlatformAvailability from the ClangImporter, we can
more easily refactor out isUnavailableInSwift from the impl, which
will free us up to do more flexible import naming.

Introduces new files ClangAdapter.h/cpp, which will serve as a
convenient place to put code reasoning about Clang details. Refactors
out most Clang-related is*, has*, and get* methods from the
ImporterImpl. In the future, an adapter class could help serve to
seperate the concerns of the importer from the details of how to
correctly use Clang APIs.

Introduces new files ClangAdapter.h/cpp, which will serve as a
convenient place to put code reasoning about Clang details. Refactors
out most Clang-related is*, has*, and get* methods from the
ImporterImpl. In the future, an adapter class could help serve to
seperate the concerns of the importer from the details of how to
correctly use Clang APIs.

Pull off the caching code from the Impl and onto a new EnumInfoCache
class, that will be useful as we decouple import naming from the Impl
class state.

Delete stray comment

static-ize more methods

Resolves SR-NNNN.

[milseman]

@swift-ci please smoke test

[milseman]

flaky LLDB

[milseman]

@shahmishal I thought LLDB tests were disabled?

[shahmishal]

Linux smoke test still has LLDB test enabled, is this failure related to this change?

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Issue Details
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ERROR: [EXCEPTIONAL EXIT 6 (SIGABRT)] <no_running_test_method> (lang/cpp/stl/TestSTL.py)

Master Linux LLDB bot has been passing, https://ci.swift.org/job/oss-lldb-incremental-linux-ubuntu-14_04/

[milseman]

Unless horribly misnamed (definitely a possibility), I don't see how a test named "lang/cpp/stl/TestSTL.py" would be relevant to the Swift project.

[milseman]

@swift-ci please smoke test Linux Platform