Support SOURCE_DATE_EPOCH environment variable

[emaste]

Summary

SOURCE_DATE_EPOCH specifies a UNIX timestamp (number of seconds since 01 Jan 1970 00:00:00 UTC) to be used as the timestamp input for build processes e.g. __DATE__ and __TIME__

See https://reproducible-builds.org/specs/source-date-epoch/ for details, and GCC support at https://gcc.gnu.org/onlinedocs/cpp/Environment-Variables.html.

[emaste]

Use gmtime to report in UTC when SOURCE_DATE_EPOCH is set. This follows GCC and makes sense given the purpose of SOURCE_DATE_EPOCH.

For reference here is discussion on the GCC list when this was introduced there: https://gcc.gnu.org/ml/gcc-patches/2015-06/msg02210.html

[emaste]

Abandon in favour of D23934

lib/Lex/PPMacroExpansion.cpp

   MacroExpandingLexersStack.pop_back();
 }
 
 /// ComputeDATE_TIME - Compute the current time, enter it into the specified
 /// scratch buffer, then return DATELoc/TIMELoc locations with the position of
 /// the identifier tokens inserted.
 static void ComputeDATE_TIME(SourceLocation &DATELoc, SourceLocation &TIMELoc,
                              Preprocessor &PP) {
-  time_t TT = time(nullptr);
+  time_t TT;
+  const char *envValue = getenv("SOURCE_DATE_EPOCH");
+  if (envValue != nullptr)
+    TT = strtol(envValue, nullptr, 10);
+  else
+    TT = time(nullptr);
   struct tm *TM = localtime(&TT);
 
   static const char * const Months[] = {
     "Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"
   };
 
   {
     SmallString<32> TmpBuffer;