[docs] TableGen easter egg: Multiline string literals

They're called code fragments, but they are really multiline string
literals. Just spotted this usage in a patch by Aaron using "code
fragments" for holding documentation text. I remember someone bemoaning
the lack of multiline string literals in TableGen, so I'm explicitly
documenting that code fragments are multiline string literals.

Let it be known that any use case needing multiline string literals in
TableGen (such as descriptions of options, or whatnot) can use use
code fragments (instead of C-style string concatenation or exceedingly
long lines). E.g.

    class Bar<int n>;
    class Baz<int n>;
    class Doc<string desc> {
        string Desc = desc;
    }
    def Foo : Bar<1>, Baz<3>, Doc<[{
    This Foo is a Bar, and also a Baz. It can take 3 values:
        * Qux
        * Quux
        * Quuux
    }]>;

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201033 91177308-0d34-0410-b5e6-96231b3b80d8
[docs] [tblgen] There is no "code" type.

Code fragments are just fancy string literals.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201034 91177308-0d34-0410-b5e6-96231b3b80d8
[docs] [tblgen] clarify that code fragments are just string literals

Fun fact: looking at the TableGen code (around TGParser.cpp:1166), the
only difference in handling is that adjacent regular string literals are
concatenated in the parser.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201035 91177308-0d34-0410-b5e6-96231b3b80d8