[Support] Move StringExtras.h include from Error.h to Error.cpp

Move the implementation of the `toString` function from
`llvm/Support/Error.h` to the source file, which allows us to move
`#include "llvm/ADT/StringExtras.h"` to the source file as well.

As `Error.h` is present in a large number of translation units this
means we are unnecessarily bringing in the contents of
`StringExtras.h` - itself a large file with lots of includes - and
slowing down compilation.

Also move the `#include "llvm/ADT/SmallVector.h"` directive to the
source file as it's no longer needed, but this does not give as much of
a benefit.

This reduces the total number of preprocessing tokens across the LLVM
source files in lib from (roughly) 1,920,413,050 to 1,903,629,230 - a
reduction of ~0.87%. This should result in a small improvement in
compilation time.

Differential Revision: https://reviews.llvm.org/D154763

llvm/include/llvm/Support/Error.h

@@ -14,8 +14,6 @@
 #define LLVM_SUPPORT_ERROR_H
 
 #include "llvm-c/Error.h"
-#include "llvm/ADT/SmallVector.h"
-#include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Config/abi-breaking.h"
 #include "llvm/Support/AlignOf.h"
@@ -1025,13 +1023,7 @@ void logAllUnhandledErrors(Error E, raw_ostream &OS, Twine ErrorBanner = {});
 
 /// Write all error messages (if any) in E to a string. The newline character
 /// is used to separate error messages.
-inline std::string toString(Error E) {
-  SmallVector<std::string, 2> Errors;
-  handleAllErrors(std::move(E), [&Errors](const ErrorInfoBase &EI) {
-    Errors.push_back(EI.message());
-  });
-  return join(Errors.begin(), Errors.end(), "\n");
-}
+std::string toString(Error E);
 
 /// Consume a Error without doing anything. This method should be used
 /// only where an error can be considered a reasonable and expected return

llvm/lib/Support/Error.cpp

@@ -7,6 +7,8 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/Error.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <system_error>
@@ -70,6 +72,15 @@ void logAllUnhandledErrors(Error E, raw_ostream &OS, Twine ErrorBanner) {
   });
 }
 
+/// Write all error messages (if any) in E to a string. The newline character
+/// is used to separate error messages.
+std::string toString(Error E) {
+  SmallVector<std::string, 2> Errors;
+  handleAllErrors(std::move(E), [&Errors](const ErrorInfoBase &EI) {
+    Errors.push_back(EI.message());
+  });
+  return join(Errors.begin(), Errors.end(), "\n");
+}
 
 std::error_code ErrorList::convertToErrorCode() const {
   return std::error_code(static_cast<int>(ErrorErrorCode::MultipleErrors),