Parser: Register all availability macro definition buffers before parsing

The availability macro definitions from the command line are copied into
source buffers for parsing. Let's register all of these buffers in one go,
before starting to parse any of them. Parsing triggers using `findBuffer`
services which sorts all buffers. The new logic should ensure this
sorting happens only once for availability macros.

Author: xymus

lib/Parse/ParseDecl.cpp

@@ -2120,11 +2120,18 @@ AvailabilityMacroMap &Parser::parseAllAvailabilityMacroArguments() {
   SourceManager &SM = Context.SourceMgr;
   LangOptions LangOpts = Context.LangOpts;
 
+  // Allocate all buffers in one go to avoid repeating the sorting in
+  // findBufferContainingLocInternal.
+  llvm::SmallVector<unsigned, 4> bufferIDs;
   for (StringRef macro: LangOpts.AvailabilityMacros) {
+    unsigned bufferID = SM.addMemBufferCopy(macro,
+                                            "-define-availability argument");
+    bufferIDs.push_back(bufferID);
+  }
 
+  // Parse each macro definition.
+  for (unsigned bufferID: bufferIDs) {
     // Create temporary parser.
-    int bufferID = SM.addMemBufferCopy(macro,
-                                       "-define-availability argument");
     swift::ParserUnit PU(SM, SourceFileKind::Main, bufferID, LangOpts,
                          TypeCheckerOptions(), SILOptions(), "unknown");