Vulkan: Adding custom pool allocator

Copied pool allocator used by compiler to common and hooking it up as
custom allocator for CommandPools. Modified it to support reallocation.

RendererVk now has a private poolAllocator and VkAllocationCallbacks
struct. The allocation callbacks are initialized to static functions
in RendererVk::initializeDevice() and then passed to CommandPool init()
and destroy() functions.

Using the pool allocator saves Command Pool/Buffer clean-up time which
was showing us as a bottleneck is some cases.

Bug: angleproject:2951
Change-Id: I81aa8a7ec60397676fa722d6435029db27947ef4
Reviewed-on: https://chromium-review.googlesource.com/c/1409867
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/compiler/translator/Compiler.cpp b/src/compiler/translator/Compiler.cpp
index 95082d9..ff6c5d0 100644
--- a/src/compiler/translator/Compiler.cpp
+++ b/src/compiler/translator/Compiler.cpp
@@ -170,7 +170,7 @@
 class TScopedPoolAllocator
 {
   public:
-    TScopedPoolAllocator(TPoolAllocator *allocator) : mAllocator(allocator)
+    TScopedPoolAllocator(angle::PoolAllocator *allocator) : mAllocator(allocator)
     {
         mAllocator->push();
         SetGlobalPoolAllocator(mAllocator);
@@ -182,7 +182,7 @@
     }
 
   private:
-    TPoolAllocator *mAllocator;
+    angle::PoolAllocator *mAllocator;
 };
 
 class TScopedSymbolTableLevel