New assertions support

This adds three new assertion macros:
* `ASSERT` - always compiled in, always checked
* `CONDITIONAL_ASSERT` - always compiled in, checked whenever the `-compiler-assertions` flag is provided
* `DEBUG_ASSERT` - only compiled into debug builds, always checked when compiled in  (functionally the same as Standard C `assert`)

The new `-compiler-assertions` flag is recognized by both `swift-frontend` and
`swiftc`.

The goal is to eventually replace every use of `assert` in the compiler with one of the above:
* Most assertions will use `ASSERT` (most assertions should always be present and checked, even in release builds)
* Expensive assertions can use `CONDITIONAL_ASSERT` to be suppressed by default
* A few very expensive and/or brittle assertions can use `DEBUG_ASSERT` to be compiled out of release builds

This should:
* Improve quality by catching errors earlier,
* Accelerate compiler triage and debugging by providing more accurate crash dumps by default, and
* Allow compiler engineers and end users alike to add `-compiler-assertions` to get more accurate failure diagnostics with any compiler

lib/Frontend/CompilerInvocation.cpp

@@ -15,6 +15,7 @@
 
 #include "ArgsToFrontendOptionsConverter.h"
 #include "swift/AST/DiagnosticsFrontend.h"
+#include "swift/Basic/Assertions.h"
 #include "swift/Basic/Feature.h"
 #include "swift/Basic/Platform.h"
 #include "swift/Option/Options.h"
@@ -328,6 +329,13 @@ bool CompilerInvocation::setModuleAliasMap(std::vector<std::string> args,
   return ModuleAliasesConverter::computeModuleAliases(args, FrontendOpts, diags);
 }
 
+static void ParseAssertionArgs(ArgList &args) {
+  using namespace options;
+  if (args.hasArg(OPT_compiler_assertions)) {
+    CONDITIONAL_ASSERT_Global_enable_flag = 1;
+  }
+}
+
 static bool ParseFrontendArgs(
     FrontendOptions &opts, ArgList &args, DiagnosticEngine &diags,
     SmallVectorImpl<std::unique_ptr<llvm::MemoryBuffer>> *buffers) {
@@ -3392,6 +3400,8 @@ bool CompilerInvocation::parseArgs(
     return true;
   }
 
+  ParseAssertionArgs(ParsedArgs);
+
   if (ParseLangArgs(LangOpts, ParsedArgs, Diags, FrontendOpts)) {
     return true;
   }