[Basic] Use StringRef::consume_front (NFC)

Author: kazutakahirata

clang/lib/Basic/Targets/AMDGPU.h

@@ -169,10 +169,8 @@ class LLVM_LIBRARY_VISIBILITY AMDGPUTargetInfo final : public TargetInfo {
     }
 
     bool HasLeftParen = false;
-    if (S.front() == '{') {
+    if (S.consume_front("{"))
       HasLeftParen = true;
-      S = S.drop_front();
-    }
     if (S.empty())
       return false;
     if (S.front() != 'v' && S.front() != 's' && S.front() != 'a') {
@@ -199,29 +197,24 @@ class LLVM_LIBRARY_VISIBILITY AMDGPUTargetInfo final : public TargetInfo {
       return true;
     }
     bool HasLeftBracket = false;
-    if (!S.empty() && S.front() == '[') {
+    if (S.consume_front("["))
       HasLeftBracket = true;
-      S = S.drop_front();
-    }
     unsigned long long N;
     if (S.empty() || consumeUnsignedInteger(S, 10, N))
       return false;
-    if (!S.empty() && S.front() == ':') {
+    if (S.consume_front(":")) {
       if (!HasLeftBracket)
         return false;
-      S = S.drop_front();
       unsigned long long M;
       if (consumeUnsignedInteger(S, 10, M) || N >= M)
         return false;
     }
     if (HasLeftBracket) {
-      if (S.empty() || S.front() != ']')
+      if (!S.consume_front("]"))
         return false;
-      S = S.drop_front();
     }
-    if (S.empty() || S.front() != '}')
+    if (!S.consume_front("}"))
       return false;
-    S = S.drop_front();
     if (!S.empty())
       return false;
     // Found {vn}, {sn}, {an}, {v[n]}, {s[n]}, {a[n]}, {v[n:m]}, {s[n:m]}

clang/lib/Basic/Warnings.cpp

@@ -199,8 +199,7 @@ void clang::ProcessWarningOptions(DiagnosticsEngine &Diags,
 
       // Check to see if this warning starts with "no-", if so, this is a
       // negative form of the option.
-      bool IsPositive = !Opt.starts_with("no-");
-      if (!IsPositive) Opt = Opt.substr(3);
+      bool IsPositive = !Opt.consume_front("no-");
 
       auto Severity = IsPositive ? diag::Severity::Remark
                                  : diag::Severity::Ignored;