Make DiagnosticIDs::getAllDiagnostics use std::vector. NFC.

The size of the result vector is currently around 4600 with
Flavor::WarningOrError, which makes std::vector a better candidate than
llvm::SmallVector.

Patch by: Andras Leitereg!

Differential Revision: https://reviews.llvm.org/D39372

llvm-svn: 321190
diff --git a/clang/lib/Basic/Diagnostic.cpp b/clang/lib/Basic/Diagnostic.cpp
index 640b42c..26baa83 100644
--- a/clang/lib/Basic/Diagnostic.cpp
+++ b/clang/lib/Basic/Diagnostic.cpp
@@ -363,7 +363,7 @@
                                           diag::Severity Map,
                                           SourceLocation Loc) {
   // Get all the diagnostics.
-  SmallVector<diag::kind, 64> AllDiags;
+  std::vector<diag::kind> AllDiags;
   DiagnosticIDs::getAllDiagnostics(Flavor, AllDiags);
 
   // Set the mapping.