docs: Reorganize landing page.

Language extensions are highly relevant to using clang as a compiler, so
move LanguageExtensions up into `Using Clang as a Compiler` on the
landing page.

The other documents from the now-gone `Language Extensions and Specs`
section on the landing page nicely fit hierarchically under
LanguageExtensions.rst, so put them under LanguageExtensions.rst's
toctree instead of on the landing page.

Impetus from Jordan Rose.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171409 91177308-0d34-0410-b5e6-96231b3b80d8
docs: Curb excessive table-of-contents depth.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171410 91177308-0d34-0410-b5e6-96231b3b80d8
Type safety attributes: add tests for enumerations (users are actually doing
this, ensure we don't regress)


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171412 91177308-0d34-0410-b5e6-96231b3b80d8
Correct Hexagon DataLayout string. Fixes bug 14744.

Patch by Krzysztof Parzyszek!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171415 91177308-0d34-0410-b5e6-96231b3b80d8
docs: Take advantage of extra level of headings.

The way Sphinx treats the "top-level" adornments is weird. It usually
uses the first top-level adornment as the page title, even if the
top-level adornment is just one "section" out of many (i.e. if the first
section is "Introduction", then it will make the page title be
"Introduction"). This behavior can be overriden by using an explicit
`..  title::` directive to set the title.

Since the Sphinx stylesheet that Clang is currently using ('haiku')
nicely puts the document title at the top of the page in the header,
this weird default behavior was resulting in a redundant "title" in the
body content. Getting rid of this redundant level of headings
effectively "exposes" one more level of heading from the stylesheet to
which now makes the real "sections" more distinct.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171417 91177308-0d34-0410-b5e6-96231b3b80d8