Revert "SERVER-29887 address clang-4.0 warnings in geo code"

This reverts commit 8204c7d.

src/mongo/db/exec/geo_near.cpp

@@ -288,7 +288,7 @@ class GeoNear2DStage::DensityEstimator {
         // Since appendVertexNeighbors(level, output) requires level < hash.getBits(),
         // we have to start to find documents at most GeoHash::kMaxBits - 1. Thus the finest
         // search area is 16 * finest cell area at GeoHash::kMaxBits.
-        _currentLevel = std::max(0, hashParams.bits - 1);
+        _currentLevel = std::max(0u, hashParams.bits - 1u);
     }
 
     PlanStage::StageState work(OperationContext* opCtx,

src/mongo/db/geo/r2_region_coverer.cpp

@@ -68,13 +68,15 @@ R2RegionCoverer::R2RegionCoverer(GeoHashConverter* hashConverter)
 R2RegionCoverer::~R2RegionCoverer() {}
 
 void R2RegionCoverer::setMinLevel(unsigned int minLevel) {
+    dassert(minLevel >= 0);
     dassert(minLevel <= GeoHash::kMaxBits);
-    _minLevel = min(GeoHash::kMaxBits, minLevel);
+    _minLevel = max(0u, min(GeoHash::kMaxBits, minLevel));
 }
 
 void R2RegionCoverer::setMaxLevel(unsigned int maxLevel) {
+    dassert(maxLevel >= 0);
     dassert(maxLevel <= GeoHash::kMaxBits);
-    _maxLevel = min(GeoHash::kMaxBits, maxLevel);
+    _maxLevel = max(0u, min(GeoHash::kMaxBits, maxLevel));
 }
 
 void R2RegionCoverer::setMaxCells(int maxCells) {

src/third_party/s2/util/math/mathutil.cc

@@ -27,13 +27,7 @@ using std::vector;
     return static_cast<IntOut>(x < 0 ? (x - 0.5) : (x + 0.5));
   }
 
-// MONGODB suppress clang-4.0 warning by disabling the following line, but leave it in for MSVC
-// since removing it results in a linker error there.
-// error: explicit instantiation of 'Round<int, double>' that occurs after an explicit
-// specialization has no effect [-Werror,-Winstantiation-after-specialization]
-#ifdef _MSC_VER
 template int MathUtil::Round<int,double>(double x);
-#endif
 
 MathUtil::QuadraticRootType MathUtil::RealRootsForQuadratic(long double a,
                                                         long double b,