[analyzer] NFC: Don't regenerate duplicate HTML reports.

This is a performance optimization for HTML diagnostics output mode.

Currently they're incredibly inefficient:

* The HTMLRewriter is re-run from scratch on every file on every report.
  Each such re-run involves re-lexing the entire file and producing
  a syntax-highlighted webpage of the entire file, with text behind macros
  duplicated as pop-up macro expansion tooltips. Then, warning and note
  bubbles are injected into the page. Only the bubble part is different
  across reports; everything else can theoretically be cached.

* Additionally, if duplicate reports are emitted (with the same issue hash),
  HTMLRewriter will be re-run even though the output file is going to be
  discarded due to filename collision. This is mostly an issue for
  path-insensitive bug reports because path-sensitive bug reports
  are already deduplicated by the BugReporter as part of searching
  for the shortest bug path. But on some translation units almost 80% of
  bug reports are dry-run here.

We only get away with all this because there are usually very few reports
emitted per file. But if loud checkers are enabled, such as `webkit.*`,
this may explode in complexity and even cause the compiler to run over
the 32-bit SourceLocation addressing limit. (We're re-lexing everything
each time, remember?)

This patch hotfixes the *second* problem. Adds a FIXME for the first problem,
which will require more yak shaving to solve.

rdar://120801986

clang/lib/StaticAnalyzer/Core/HTMLDiagnostics.cpp

@@ -68,6 +68,7 @@ class HTMLDiagnostics : public PathDiagnosticConsumer {
   bool noDir = false;
   const Preprocessor &PP;
   const bool SupportsCrossFileDiagnostics;
+  llvm::StringSet<> EmittedHashes;
 
 public:
   HTMLDiagnostics(PathDiagnosticConsumerOptions DiagOpts,
@@ -301,6 +302,17 @@ void HTMLDiagnostics::ReportDiag(const PathDiagnostic& D,
       }
   }
 
+  SmallString<32> IssueHash = getIssueHash(D, PP);
+  auto [It, IsNew] = EmittedHashes.insert(IssueHash);
+  if (!IsNew) {
+    // We've already emitted a duplicate issue. It'll get overwritten anyway.
+    return;
+  }
+
+  // FIXME: This causes each file to be re-parsed and syntax-highlighted
+  // and macro-expanded separately for each report. We could cache such rewrites
+  // across all reports and only re-do the part that's actually different:
+  // the warning/note bubbles.
   std::string report = GenerateHTML(D, R, SMgr, path, declName.c_str());
   if (report.empty()) {
     llvm::errs() << "warning: no diagnostics generated for main file.\n";
@@ -332,7 +344,7 @@ void HTMLDiagnostics::ReportDiag(const PathDiagnostic& D,
              << declName.c_str() << "-" << offsetDecl << "-";
   }
 
-  FileName << StringRef(getIssueHash(D, PP)).substr(0, 6).str() << ".html";
+  FileName << StringRef(IssueHash).substr(0, 6).str() << ".html";
 
   SmallString<128> ResultPath;
   llvm::sys::path::append(ResultPath, Directory, FileName.str());