[Support] Remove redundant declarations (NFC) (#166106)

In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

Identified with readability-redundant-declaration.

Author: kazutakahirata

llvm/lib/Support/BranchProbability.cpp

@@ -20,8 +20,6 @@
 
 using namespace llvm;
 
-constexpr uint32_t BranchProbability::D;
-
 raw_ostream &BranchProbability::print(raw_ostream &OS) const {
   if (isUnknown())
     return OS << "?%";

llvm/unittests/Support/raw_ostream_proxy_test.cpp

@@ -40,8 +40,6 @@ class BufferedNoPwriteSmallVectorStream : public raw_ostream {
   bool IsDisplayed = false;
 };
 
-constexpr size_t BufferedNoPwriteSmallVectorStream::PreferredBufferSize;
-
 TEST(raw_ostream_proxyTest, write) {
   // Besides confirming that "write" works, this test confirms that the proxy
   // takes on the buffer from the stream it's proxying, such that writes to the