[Clang Tablegen][RFC] Allow Early Textual Substitutions in `Diagnostic` messages.

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

1 files changed, 1 insertions, 1 deletions

diff --git a/clang/test/SemaCXX/anonymous-struct.cpp b/clang/test/SemaCXX/anonymous-struct.cpp
index bb7e6eb92da..f89d19959b8 100644
--- a/clang/test/SemaCXX/anonymous-struct.cpp
+++ b/clang/test/SemaCXX/anonymous-struct.cpp
@@ -16,7 +16,7 @@ struct E {
   struct {
     S x;
 #if __cplusplus <= 199711L
-    // expected-error@-2 {{anonymous struct member 'x' has a non-trivial constructor}}
+    // expected-error@-2 {{anonymous struct member 'x' has a non-trivial default constructor}}
 #endif
   };
   static struct {