[NFC] Use higher level constructs to check for whitespace/newlines in the lexer

[serge-sans-paille]

Summary

It turns out that according to valgrind and perf, it's also slightly faster.

[serge-sans-paille]

Concerning the performance aspect, when counting the number of instructions needed to compile the sqlite amalgametion, using

valgrind --tool=callgrind ./bin/clang  -c sqlite3.c -o/dev/null -w

I get 5,872,876,067 instructions after the patch, and 5,879,411,049 before.

clang/lib/Lex/Lexer.cpp

bool Lexer::LexAngledStringLiteral(Token &Result, const char *CurPtr) {
   const char *AfterLessPos = CurPtr;
   char C = getAndAdvanceChar(CurPtr, Result);
   while (C != '>') {
     // Skip escaped characters.  Escaped newlines will already be processed by
     // getAndAdvanceChar.
     if (C == '\\')
       C = getAndAdvanceChar(CurPtr, Result);
 
-    if (C == '\n' || C == '\r' ||                // Newline.
+    if (isVerticalWhitespace(C) ||               // Newline.
         (C == 0 && (CurPtr - 1 == BufferEnd))) { // End of file.
       // If the filename is unterminated, then it must just be a lone <
       // character.  Return this as such.
       FormTokenWithChars(Result, AfterLessPos, tok::less);
       return true;
     }
 
     if (C == 0) {
LexNextToken:
   // New token, can't need cleaning yet.
   Result.clearFlag(Token::NeedsCleaning);
   Result.setIdentifierInfo(nullptr);
 
   // CurPtr - Cache BufferPtr in an automatic variable.
   const char *CurPtr = BufferPtr;
 
   // Small amounts of horizontal whitespace is very common between tokens.
-  if ((*CurPtr == ' ') || (*CurPtr == '\t')) {
-    ++CurPtr;
-    while ((*CurPtr == ' ') || (*CurPtr == '\t'))
+  if (isHorizontalWhitespace(*CurPtr)) {
+    do {
       ++CurPtr;
+    } while (isHorizontalWhitespace(*CurPtr));
 
     // If we are keeping whitespace and other tokens, just return what we just
     // skipped.  The next lexer invocation will return the token after the
     // whitespace.
     if (isKeepWhitespaceMode()) {
       FormTokenWithChars(Result, CurPtr, tok::unknown);
       // FIXME: The next token will not have LeadingSpace set.
       return true;