PrintStackTrace: don't symbolize if LLVM_DISABLE_SYMBOLIZATION is set

See http://lists.llvm.org/pipermail/llvm-dev/2017-June/113975.html for a related previous discussion.
Many tools install signal handlers to print stack traces and optionally
symbolize the addresses with an external program 'llvm-symbolizer' (when
searching for 'llvm-symbolizer', the directory containg the executable
is preferred over PATH).

'llvm-symbolizer' can be slow if the executable is large and/or if
llvm-symbolizer' itself is under-optimized. For example, my 'llvm-lto2' from a
-DCMAKE_BUILD_TYPE=Debug build is 443MiB. The 'llvm-symbolizer' from the same
build takes ~2s to symbolize it. (An optimized 'llvm-symbolizer' takes 0.34s).
A crashed clang may take more than 5s to symbolize a stack trace.

If a test file has several `not --crash` RUN lines. It can be very slow in a Debug build.
This patch makes `not --crash` set an environment variable to suppress symbolization.
This is similar to D33804 which uses a command line option.
I pick 'symbolization' instead of 'symbolication' because the former is
used much more commonly and its stem matches 'llvm-symbolizer'.

Also set LLVM_DISABLE_CRASH_REPORT=1, which is currently only applicable on
`__APPLE__`.

Reviewed By: dblaikie, aganea

Differential Revision: https://reviews.llvm.org/D86170

llvm/lib/Support/Signals.cpp

@@ -44,6 +44,8 @@ static cl::opt<bool, true>
                          cl::desc("Disable symbolizing crash backtraces."),
                          cl::location(DisableSymbolicationFlag), cl::Hidden);
 
+constexpr char DisableSymbolizationEnv[] = "LLVM_DISABLE_SYMBOLIZATION";
+
 // Callbacks to run in signal handler must be lock-free because a signal handler
 // could be running as we add new callbacks. We don't add unbounded numbers of
 // callbacks, an array is therefore sufficient.
@@ -105,7 +107,7 @@ static FormattedNumber format_ptr(void *PC) {
 LLVM_ATTRIBUTE_USED
 static bool printSymbolizedStackTrace(StringRef Argv0, void **StackTrace,
                                       int Depth, llvm::raw_ostream &OS) {
-  if (DisableSymbolicationFlag)
+  if (DisableSymbolicationFlag || getenv(DisableSymbolizationEnv))
     return false;
 
   // Don't recursively invoke the llvm-symbolizer binary.

llvm/test/tools/not/disable-symbolization.test

@@ -0,0 +1,5 @@
+# RUN: not --crash printenv > %t || true
+# RUN: FileCheck %s < %t
+
+# CHECK-DAG: LLVM_DISABLE_CRASH_REPORT=1
+# CHECK-DAG: LLVM_DISABLE_SYMBOLIZATION=1

llvm/utils/not/not.cpp

@@ -15,6 +15,10 @@
 #include "llvm/Support/WithColor.h"
 #include "llvm/Support/raw_ostream.h"
 
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
 using namespace llvm;
 
 int main(int argc, const char **argv) {
@@ -27,6 +31,16 @@ int main(int argc, const char **argv) {
     ++argv;
     --argc;
     ExpectCrash = true;
+
+    // Crash is expected, so disable crash report and symbolization to reduce
+    // output and avoid potentially slow symbolization.
+#ifdef _WIN32
+    SetEnvironmentVariableA("LLVM_DISABLE_CRASH_REPORT", "1");
+    SetEnvironmentVariableA("LLVM_DISABLE_SYMBOLIZATION", "1");
+#else
+    setenv("LLVM_DISABLE_CRASH_REPORT", "1", 0);
+    setenv("LLVM_DISABLE_SYMBOLIZATION", "1", 0);
+#endif
   }
 
   if (argc == 0)