Fail compilation on sources larger than 2GiB

Glslang takes the source length as an int. Fail compilation
if the string is so large that conversion from the length
to int would overflow.

Google-internal bug: 536201886

libshaderc_util/src/compiler.cc

diff --git a/libshaderc_util/src/compiler.cc b/libshaderc_util/src/compiler.cc
index a5b6131..bbd5469 100644
--- a/libshaderc_util/src/compiler.cc
+++ b/libshaderc_util/src/compiler.cc
@@ -14,6 +14,7 @@
 
 #include "libshaderc_util/compiler.h"
 
+#include <climits>
 #include <cstdint>
 #include <iomanip>
 #include <sstream>
@@ -195,6 +196,16 @@
   std::vector<uint32_t>& compilation_output_data = std::get<1>(result_tuple);
   size_t& compilation_output_data_size_in_bytes = std::get<2>(result_tuple);
 
+  // glslang takes source lengths as int; a size_t that does not fit would be
+  // truncated (negative -> glslang falls back to strlen() on a buffer that is
+  // not required to be NUL-terminated). Reject oversized inputs up front.
+  if (input_source_string.size() > static_cast<size_t>(INT_MAX)) {
+    *error_stream << error_tag
+                  << ": error: shader source exceeds INT_MAX bytes\n";
+    *total_errors = 1;
+    return result_tuple;
+  }
+
   // Check target environment.
   const auto target_client_info = GetGlslangClientInfo(
       error_tag, target_env_, target_env_version_, target_spirv_version_,