Move LangStandard*, InputKind::Language to Basic

This patch is a prerequisite for using LangStandard from Driver in
https://reviews.llvm.org/D64793.

It moves LangStandard* and InputKind::Language to Basic.  It is mostly
mechanical, with only a few changes of note:

- enum Language has been changed into enum class Language : uint8_t to
  avoid a clash between OpenCL in enum Language and OpenCL in enum
  LangFeatures and not to increase the size of class InputKind.

- Now that getLangStandardForName, which is currently unused, also checks
  both canonical and alias names, I've introduced a helper getLangKind
  which factors out a code pattern already used 3 times.

The patch has been tested on x86_64-pc-solaris2.11, sparcv9-sun-solaris2.11,
and x86_64-pc-linux-gnu.

There's a companion patch for lldb which uses LangStandard.h
(https://reviews.llvm.org/D65717).

While polly includes isl which in turn uses InputKind::C, that part of the
code isn't even built inside the llvm tree.  I've posted a patch to allow
for both InputKind::C and Language::C upstream
(https://groups.google.com/forum/#!topic/isl-development/6oEvNWOSQFE).

Differential Revision: https://reviews.llvm.org/D65562

llvm-svn: 367864