[formatv] Leave format parameters unstripped (#112625)

This is consistent with std::formatv and allows formatters to support a
wider variety of use cases (like having a bare string in their formatter
if that's useful, etc).

Came up in the context of some Carbon diagnostic work here:
carbon-language/carbon-lang#4411 (comment)

llvm/lib/Support/FormatVariadic.cpp

@@ -64,21 +64,20 @@ static std::optional<ReplacementItem> parseReplacementItem(StringRef Spec) {
   AlignStyle Where = AlignStyle::Right;
   StringRef Options;
   unsigned Index = ~0U;
-  RepString = RepString.trim();
+  RepString = RepString.ltrim();
 
   // If index is not specified, keep it ~0U to indicate unresolved index.
   RepString.consumeInteger(0, Index);
-  RepString = RepString.trim();
 
   if (RepString.consume_front(",")) {
     if (!consumeFieldLayout(RepString, Where, Align, Pad)) {
       assert(false && "Invalid replacement field layout specification!");
       return std::nullopt;
     }
   }
-  RepString = RepString.trim();
+  RepString = RepString.ltrim();
   if (RepString.consume_front(":")) {
-    Options = RepString.trim();
+    Options = RepString;
     RepString = StringRef();
   }
   RepString = RepString.trim();

llvm/unittests/Support/FormatVariadicTest.cpp

@@ -150,7 +150,7 @@ TEST(FormatVariadicTest, ValidReplacementSequence) {
   EXPECT_EQ(0u, Replacements[0].Index);
   EXPECT_EQ(3u, Replacements[0].Width);
   EXPECT_EQ(AlignStyle::Left, Replacements[0].Where);
-  EXPECT_EQ("foo", Replacements[0].Options);
+  EXPECT_EQ(" foo ", Replacements[0].Options);
 
   // 8. Everything after the first option specifier is part of the style, even
   // if it contains another option specifier.