[flang] Accept multiple spaces after compiler directive sentinel (llvm#76541)

The prescanner allows multiple spaces within a compiler directive, but
not between the directive's sentinel (e.g., !DIR$) and the directive's
first token.

Fixes llvm#76537.

Author: klausler

flang/lib/Parser/prescan.cpp

@@ -139,16 +139,18 @@ void Prescanner::Statement() {
         SkipSpaces();
       }
     } else {
-      // Compiler directive.  Emit normalized sentinel.
+      // Compiler directive.  Emit normalized sentinel, squash following spaces.
       EmitChar(tokens, '!');
       ++at_, ++column_;
       for (const char *sp{directiveSentinel_}; *sp != '\0';
            ++sp, ++at_, ++column_) {
         EmitChar(tokens, *sp);
       }
-      if (*at_ == ' ') {
+      if (*at_ == ' ' || *at_ == '\t') {
         EmitChar(tokens, ' ');
-        ++at_, ++column_;
+        while (*at_ == ' ' || *at_ == '\t') {
+          ++at_, ++column_;
+        }
       }
       tokens.CloseToken();
     }

flang/test/Parser/compiler-directives.f90

@@ -17,6 +17,7 @@ module m
   !dir$ integer
   !dir$ integer=64
   !dir$ integer = 64
+  !dir$  integer = 64
   PROC(4)
   !dir$ optimize:1
   !dir$ optimize : 1