[libomptarget][NFC] Avoid gcc 5/6 issue with lambda captures.

[vzakhari]

Summary

See discussion in https://reviews.llvm.org/D93727#inline-893326

[grokos]

LGTM, thanks!

openmp/libomptarget/include/Debug.h

enum OpenMPInfoType : uint32_t {
   // Dump the contents of the device pointer map at kernel exit or failure.
   OMP_INFOTYPE_DUMP_TABLE = 0x0004,
   // Print kernel information from target device plugins.
   OMP_INFOTYPE_PLUGIN_KERNEL = 0x0010,
   // Enable every flag.
   OMP_INFOTYPE_ALL = 0xffffffff,
 };
 
-static inline uint32_t getInfoLevel() {
+// Add __attribute__((used)) to work around a bug in gcc 5/6.
+static inline uint32_t __attribute__((used)) getInfoLevel() {
   static uint32_t InfoLevel = 0;
   static std::once_flag Flag{};
   std::call_once(Flag, []() {
     if (char *EnvStr = getenv("LIBOMPTARGET_INFO"))
       InfoLevel = std::stoi(EnvStr);
   });
 
   return InfoLevel;
 }
 
-static inline uint32_t getDebugLevel() {
+// Add __attribute__((used)) to work around a bug in gcc 5/6.
+static inline uint32_t __attribute__((used)) getDebugLevel() {
   static uint32_t DebugLevel = 0;
   static std::once_flag Flag{};
   std::call_once(Flag, []() {
     if (char *EnvStr = getenv("LIBOMPTARGET_DEBUG"))
       DebugLevel = std::stoi(EnvStr);
   });
 
   return DebugLevel;