Summary:
There are cases where the same string or select is repeated verbatim in a lot of diagnostics. This can be a pain to maintain and update. Tablegen provides no way stash the common text somewhere and reuse it in the diagnostics, until now!
This patch allows diagnostic texts to contain `%sub{<definition-name>}`, where `<definition-name>` names a Tablegen record of type `TextSubstitution`. These substitutions are done early, before the diagnostic string is otherwise processed. All `%sub` modifiers will be replaced before the diagnostic definitions are emitted.
The substitution must specify all arguments used by the substitution, and modifier indexes in the substitution are re-numbered accordingly. For example:
```
def select_ovl_candidate : TextSubstitution<"%select{function|constructor}0%select{| template| %2}1">;
```
when used as
```
"candidate `%sub{select_ovl_candidate}3,2,1 not viable"
```
will act as if we wrote:
```
"candidate %select{function|constructor}3%select{| template| %1}2 not viable"
```
Reviewers: rsmith, rjmccall, aaron.ballman, a.sidorin
Reviewed By: rjmccall
Subscribers: cfe-commits
Differential Revision: https://reviews.llvm.org/D46740
llvm-svn: 332799
b87be18d8e
